This happened to me and I restarted my iPhone only for the timer to restart to 3 hours and then eventually get stuck again. If you take it to the Apple store they’ll usually tell you to just wipe your phone and restore the latest iCloud backup you have, but I didn’t back up my phone so I tried to restart my phone again and wait for the whole 3 hours to go by without turning my phone on at all to check the timer, and after the 3 hours passed, it became available.