Minecraft is constantly and continually crashing to the point where I can't play.

When I launch minecraft, it starts up perfectly normally. when I go to log into a world, as soon as it gets to the point where I'm dropped into the world, the screen goes black, but the hotbar stays for a split second before the whole window closes and the launcher pops up with no log or error report open. this is new, as minecraft was running fine this morning.