I cant move

Why is it that in some games, when my opponant moves, and I go to block them -the piece I plan to block them with won't move. The opponant piece is highlighted in yellow. It happened with their queen and knight. It makes me unable to play at all.

your king is likely in check. post a link to the game if you want someone to investigate. 

http://www.chess.com/livechess/game.html?id=10514826

I hope this link works, when their queen moved to position 12, I could not get my pawn to move. When their knight moved to position 13, I had trouble moving pieces toward it. I do not know what the problem is.

So I suppose my king is in check even though they can't get him for another couple of moves? And even though there is a way to prevent this from happening, because it is in "check" through a computer system, I have to just sit there and wait for them to kill me without being able to stop it?

Which pawn were you trying to move on move 12 and where were you trying to move it to ?