My Invetory is messed up

I was on mulitplayer and i was about to build and when i clicked on a block in my inventory it didnt work.I would have to hold onto it to carry it.Then i tried on singleplayer and the same thing happend.Then i tried to put the blocks back in my inventory where you get them and it wont even go in to it.

it's really on 1.6.2