Stuck on Lock Screen and nothing would work :(

[officially_juju]

So I pulled my phone out , turned on the lock screen and... NOTHING :angry: It was stuck, taunting me. None of the side or capacitive buttons would even work. I tried many soft resets and finally minutes later it rebooted.
What causes this annoyance!? :shocked:

 

[jomargon]

The same thing happened to me when I was in an area with no LTE coverage.