xti 400d shutter won't trip

I cannot get the shutter on my EOS xti 400d to trip when my lens ( EF-S 18-55mm) is set on autofocus. It works fine with manual focus. What's going on? (In case my question didn't make it plain, I am a total amateur...) Thanks.

did you get the focus confirmation beep? It may be that the camera simply won't take a picture until it has locked focus.

With your clue I determined that it is in low light situations that the shutter doesn't work. But the built-in flash is activated, so why doesn't it kick in to enable the exposure?

If the camera can't lock focus it won't shoot. The onboard flash is a strobe, not a flashlight. The light duration is measured in milliseconds. If you had an external flash that offered an AF assist beam then that would work. The AF assist beam is a red grid projected onto the subject to allow the AF points to find some contrast to lock onto. Point and shoot cameras tend to have an AF assist beam to help them focus; of course, the depth of field is so deep with the small sensors of point and shoots and camera phones that even if it misses focus it doesn't matter.