Widget, Welcome to the Forum and Thanks for joining us, we are happy to have you.

Have you tried removing the battery for a few minutes, and then restarting the phone? Also try turning off the phone and hooking it to the AC charger for a several minutes and disconnecting and then starting the phone.

If neither of these help then turn off the phone and hold down the Caps Shift(lower left, up arrow), Space Bar(lower right space), Delete Bar(upper right, left pointing arrow) and the On Button(power), all at the same time, until you see the handshake screen, and release. This will do a hard reset of your phone, so I hope that you have made a backup of it. If not and one of the first remedies solves the problem, please take this opportunity to make a backup of your phone to your PC, as you never know when it will be needed.

I hope this helps and that you have a good day/

I had this issue too. Kept doing the 4 finger salute to get it back up, but issue kept returning.

Turned out to be the memory card. Just removed it before turning on and voila!