Flash does not fire - Canon EOS 500D / Digital Rebel T1i Digital Camera

Does flash pop up? Does the little electronic flash symbol appear in the LCD when it charges? Really not enough info in your post.

A few thoughts:

If the flash will not pop up check the external flash detection pin in the hot shoe (located on one side under the spring, which can be popped off with a knife blade or similar). If this is stuck down the flash will not pop up because the camera thinks there is an external flash in the hot shoe.

If the flash pops up and you get an 05 error or it makes a clicking noise and you get an 05 error this usually means sticky liquid spillage around the flash. It needs to be disassembled and cleaned and the pivots lubed. Squirting WD40 on the flash WILL NOT SOLVE THIS PROBLEM. NEVER SQUIRT WD40 IN ANY CAMERA!!!!!

It the flash pops up, and the little electronic flash symbol comes on, but goes out when you push the release button it usually means the flash board is bad.

If the camera has been impacted, then either the flash tube is cracked/broken or one of the wires that goes to it or the trigger coil may be broken inside