This is mostly a plug in problem, to easily figure out which one is doing it use the command and conquer technique.

Move half of the plugins in the  wp-contentplugins to a different folder on your server.  (so basically move the complete plug ins folders)

Check if you can log in.

If you can’t move half of the remaining and so on.

As soon as you can log into WordPress admin section, take back the batch that was causing the issue and do a command and conquer on it (i.e take half of the plug in out of that group and test and so on)

 
What causes this is a stuck power button.

My suggestion is to open up the phone (look up a s4 teardown)  (This is a good one  https://www.youtube.com/watch?v=zm4f2XjQLAM)

You only need to remove the bezel, after that you will see the actual power button.  Wiggle it until gets unstuck.

If this helped you, please put in a comment.

THANKS!