Camera won't auto focus anymore?

I have a nikon d40. I've had the same lense 18-55 on it since my friend gave it to me in September. I shoot at raves mainly and recently this passed weekend my camera won't auto focus anymore. What's happening? And yes the lense it switch to the a not the m.

Added (1). As in an external flash? Yes. I don't use the built in flash anymore.

Somethings got to be broken… Either that or you might have a manual button you have to press to turn on auto focus.

You may want to visit a proper camera store and have them look at it. Try using another AF-S lens while there and see if the same problem occurs. If it does, it could be as simple as the electronic contacts being dirty, either on the lens, the camera or both

There are times that lenses will not auto-focus. Usually it has its genesis in attempting to shoot under very low light conditions.

Are you using a one of the Nikon electronic flash units?