Help! My shower won't turn off!

For a while my shower has been dripping. In order to get it to stop dripping we had to turn the handles really hard (I know this is bad, but it was either that or listen to a dripping shower all night). Just now when I went to turn off the hot water, it gushed suddenly instead of slowing down, then when I turned the handle more it slowed, but now the handle is as tight as I can make it and the water is still completely ON! It’s running at about half its normal pressure – i.e., WAY more than just a drip! What do I do? Is there a way to turn off the water at its source somewhere? Tomorrow I can call a plumber but it’s 8:30 at night! Ack!