Frozen while charging, is this my phone or the focus?

[Neibl]

Frequently when my phone is charging for long periods while I am sleeping, I wake up and the screen will not turn on, most likely frozen in the lock screen with the screen off. I hold the power button for 20 seconds and then it shuts off and the phone restarts normally.

What is the reason for this? I get feeling my charger might be bad (i hope that is it), but I recently had the phone fixed by samsung and it does reset randomly everyone once in a while, not uncommon in my surround though either (probably less though).

Any thoughts?