What is wrong with my camera's flash?
My camera's flash isn't working. It pops up normally, but the flash doesn't go off when I take a picture.
The shutter sensor pin may be stuck. The fix for this can be found on the Nikon D5000 troubleshooting page, among other possible solutions. If you've used the flash several times consecutively, the camera may be automatically disabling the flash temporarily to protect it.