I have a Nikon D40 adn the built in flash pops up, but does not flash when pictures are taken.

SOURCE: Flash won't fire

If it clicks when the flash is already open, it seems there would be a problem with the flash open sensor. The camera thinks the flash is closed, so it tries to open it again and it refuses to fire, because firing in the closed position would blow the bulb.

Here's a <a href='http://photo.net/bboard/uploaded-file?bboard_upload_id=42254384'>photo of the sensor in a D70</a>. The D40 flash is a bit different, but if you open it (just unscrew the two little screws in front) you'll recognize the sensor clips. Open and close the flash and see if the make contact when open.

(And DON'T fire the flash when open, or at the very least don't touch anything in there when firing or you'll get zapped.

SOURCE: Nikon D40 will not flash

The lightning bolt icon in viewfinder blinks when flash is not ready, this can happen if flash doesnt pop up all the way, or if flash strobe tube is bad.

SOURCE: Nikon D40 help?!

I assume the lens had the impact. The 18-55 lens has the built-in motors for auto-focus.
Try switching to manual focus (on the lens). If this works, the lens may not auto-focus anymore.
Try it and let me know.
cheers,

SOURCE: nikon d40 flash won't fire

My d40 did the same thing and the guy at Wolf Camera said a sensor is bad. The falsh was up but every time I focused it tried to pop up the flash again but it would not fire.

I had 2 options. Send it off for 8 weeks to Nikon at a $240 minimum or buy a flash attachment for $100. I chose the $100 option.

SOURCE: nikon d40 built in flash not firing but it popped up

there is a sensor in there that tells the camera the flash is up you need to take it to a repair shop its not a at home thing