I have tried rebooting the hub by taking the power cable out and reinserting it but this didn't resolve the issue. I've also tried reinstalling the android app.
Is the light on the hub green? If so Press the network reset switch on the bottom of the hub for 7 seconds or more with a tooth pick or paperclip. It will be the only round hole out of all the square ones. Wait until the hub light turns green again.
Once it does, from a PC go to http://hubitat.local and see if you can get in (NOT HTTPS)
If you do note the ip address and put a reservation in your router.
If you can't get in go to hubitat.local:8081 and do a soft reset and restore.