Try the following and see if this resolves it...
- Press and hold the "Power" button for about 15 seconds. The phone should vibrate and restart.
- Power off and then back on (fully off) the phone.
- Power off the phone and remove the battery for about 2 minutes. Replace the battery and then power on the phone.
- Try using some apps that can be found online to exit "Fast Boot". Note, there is no guaranty that they will work.
Since you likely rooted the phone prior to this, and if the fixes listed do not work, you may well have to look into unrooting the phone before you can resolve the problem.