Boot stuck on grey screen

[vmachiel]

Hi Forum,

Since this morning my MBP (specs in sig) will only display a grey screen when I try to boot it, and nothing else: no Apple logo, no spinning beach ball. I can't do anything. I've tried resetting PRAM and SMC, hold cmd+r, hold command+c, boot safe mode, hold option, and tried booting of a recovery USB I made.
I've even tried swapping the RAM I've upgraded back the the apple branded 4GB. Nothing works.

Is it time to take it to my local Apple store, or is there something I can do?

Thank you.