My hub is a c8 i have had it for a few months now and have mostly gotten to know it.
About 2 months ago i started to get unable to connect to hub messages. With everyones help i was able to switch my router to a dedicated port for the hub, the problem went away until last week and again yesterday. I am unable to reconnect without powering down the hub and back up. Rebooting the router and unplugging and replugging the network cable helps.
Gary Toffelmire
Are you using a static ip or reservation? Have you looked at your logs?
It's a static IP
No I have not looked at logs. I don't understand the them when I do.
By that do you mean an IP set on the HE hub alone? Typically it is recommended to reserve an IP on your router.
It is a static IP on my router
That's a reserved IP not a static. Which is preferred. When you get the you cannot connect are you trying from the app or from a PC. It it fails on one does it fail on the other?
I do not connect from a pc but it fails from all of our devices (all Android)
Gary
Can you test from a web browser on a pc/laptop/mac?
I don't mean to criticize, but when you say "I was able to switch my router to a dedicated port for the hub", which I assume means you reserved a DHCP address for the hub, it makes me wonder if the reservation process was not done properly.
Your hub may very well be online and accessible, using a different IP address.
You can use https://findmyhub.hubitat.com/ to look for your hub at a different address, or an Android app like Fing or Network Analyzer to scan your network.
Also, if you plan on using a reserved DHCP address, the settings within the Hubitat must be reset to default. That is done by pressing the reset button on the bottom of the unit.
I suggest you perform the reset immediately, before trying anything else. All of our advice will be more useful if we know you are starting from a known state.
I also dont mean to be snarcky but you have to appreciate that i am not an expert in this field.
When i set the router to the reserved dhcp address i was working under the guidance of the great people here in the community, it has worked for about 2 months so i assumed that i had correctly followed the steps.
I have asked my tablet to find my hub using the find hubs option with no result.
I will setup so that i can see if my very old pc can find it next time it fails.
I will try to set my very old pc (windows) up for the next time i get the failure.
This entire post is missing the point.
This text I quoted right here is telling me this is not an IP issue. It sounds more like the hub is going offline. The DHCP and IP are probably fine, hence why a restart of the hub fixes the problem. When you are not able to connect to the hub, did you test any automations that use z-wave devices? Or another thing you can do is check the logs for any sign of activity while the hub was unreachable. No activity during that time probably means the hub crashed and was offline for that reason.
That is certainly a possibility. The OP should advise us on the color of the light, and, as you suggested, prove if the hub is actually working by testing one of his automations.
Since it does work when the cable is simply unplugged an re-plugged, is why I suspect the hub is simply switching from one IP to another.
As you know, a device will often be given the same address even without a reservation being made, during the DHCP lease time. I am thinking that as long as the connection is stable at the time of lease expiry, he will obtain the same address, and the hub can run normally for months as the OP described. But if the hub and router are both rebooted at the same time, he could be given another address that he is not aware of.
My theory is based on the assumption that he hasn't set up the DHCP reservation properly, and of course I could be wrong. 
.
I suggest the OP visit https://my.hubitat.com, then select My Hub's, and then click on Hub Details. Oserve the Last Check in time as an optional method of proving if the hub is operating on another address.
Ok this was convenient, it just stopped so i made notes.
There is no activity when i trigger an automation
I can't connect from my pc (no surprise it is only on Wifi)
The ethernet cable has a blinking green light at the router
Hub has a green light
Router reboot no change
Ethernet cable unplug plug no change
Press the hub reset button no change
Power hub down up hub light comes on blue and after a wait turns green
System back on.
So it does look like the hub has crashed.
Now what? A warranty replacement? It is not really a viable system with intermittent failures.
Gary
Usually this type of issue is software / device related, but hard to track down. It can especially happen if you have a lot of LAN/IP devices using the network stack on the hub.
The first thing to rule out if Jumbo Frames, if you have any sort of servers or NAS on your LAN sometimes it gets enabled to increase transfer speeds. Multicast jumbo frames for some reason knock out the hubs networking and makes it crash. It could be enabled on the router itself, or even just on a single device. If you have any old "dumb" switches laying around those will usually block JF so putting it behind one can rule out that as a possible issue. Also you could switch the hub to Wifi, since JF multicast would not reach the hub on Wifi.
Jumbo frames? Thats a new one to me, i asked dr google and it seems that i would have to enable it. Don't know.
We are just two old people with very light internet use, tv email surfing.
When it went off this evening there was nothing happening on our network.
I guess i could switch to wifi but i have always considered ethernet to be better.
Gary
Ethernet connection would be better, its just a test to see if it makes a difference.
This is the info that is typically useful when trying to figure out these cases
Switching to Wi-Fi would be a quick way to determine if your hub is actually failing. Once that is determined, we can go back and diagnose the Ethernet problem.
I have switched to wifi and there was a software update that i installed.
I guess it is a waiting game again.
Is there anything else i should do?
Gary
The system has shut down again.
The only change is that i have switched to wifi from ethernet (cable unplugged).
It was working normally until it stopped doing automation and communicating. A power off/on restored operations.
Do i need to start the process to get it replaced? I feel the system is unusable as it is.
Gary