I've had to give the HE app the IP address of my hub to help it find the hub lately. It has a static IP thru my router. I also noticed there's an exclamation point in the app. I did a reboot but that didn't help.
You might also try rebooting the router and then checking the router settings to be sure the Hubitat is still using the same static IP address. A connection failure could occur on either end of the Ethernet cable.
Well, it's working better, but I still have the exclamation point. Not sure what that means.
The exclamation point isn't a problem. It is the integrated browser in the app flagging the HE built in site as not using SSL/TLS to create a secure connection.