I just unboxed my C8, updated the firmware and rebooted. Once I came online I setup a user and when I pushed save the device went offline and has not come back.
I did a hard reboot, but that has not resolved the issue.
What color is the LED on the front of the hub? Green is good; anything else (or nothing at all) could be a problem.
If it's green, could your hub's IP address have changed between reboots? Use findmyhub.hubitat.com to check. If so, you can avoid this by setting a DCHP reservation on your router/gateway, or setting a static IP and associated settings on the hub itself (I'd recommend the former).