Stage one: DENIAL
Just stare from where the incident took place with a dumbfounded look.

Stage 2: ANGER
I would mess up my whole room screaming my head and bite into my pillows shaking them like a rabid dog. 

Stage 3 Bargaining:
  I WOULD BEG AND BEG AND BEG BEG..."I WONY DO IT AGAIN JUST PLEASE LET ME HAVE MY GAMES BACK!!!"

Stage 4: depression:
I'd sit in my room staring at a wall hugging my knees probably crying my head off....

Stage 5: Anger AGAIN
Mess up my room again...

Stage 6 FAQ U DEN...
   Stay mad AT Slenderman, not talking to him or noticing  that's he's there, so flat out pretending he dosent exist...

Stage 7: Get my flipping games back..
Finally I would come to my senses and just go get my games back...

Yea that's exactly what would happen...