My Mac Book Pro won't let me drag anything?!?

For some reason all of a sudden one day my trackpad won't let me drag things around to the desktop or anywhere else. Therefore, it won't let me highlight or anything!! I've had the Mac book Pro for a year and a half and have no idea how to fix this problem. It won't even let me make programs smaller or larger because I can't drag!! please tell me how to fix this!!