cube crash

[Chungo]

So i yesterday i got portal 2 and i played it for a little wile the my freind came on and we did coop. so the next day i went on to play singleplayer and i picked up a cube and it crashed on me. So i went back on and i tried to pick up the cube and put it down again and it did the same thing. Does anyone have a anwser to this because portal 2 is awsome and i really want to finish it.

P.S i have windows 8 64 bit