flash ,on canon eos 400D.

faurie, a quick search reveals this to be a common error. you might try what was suggested on here " Here’s the fix to the err05 msg when the flash does pop-up. This will work on the XTi and perhaps on similar Canon cameras:

The pop-up flash has two legs. Look at the pivot point of the right leg (”right” when looking into the lens). Just in front of the pivot is a very small spring-load pin on the “roof” of the camera body.

This pin is the sensor that tells the camera the state of the flash. The pin is pushed down when the pop-up flash is closed and is suppose to quickly spring up when the flash is opened. Problem is, the pin sometimes gets dirty and drags. When it is slow to spring up … you get the err05 message.

Fix: Take a small jeweler’s screwdriver and dip the tip in isopropyl alcohol. (Don’t use any other solvent unless it’s designed for cleaning electronic parts!) Then touch the screwdriver tip to the pin. Do this several times until the pin area is nicely wet. Then push the pin up and down until it is able to spring up quickly." Hope this helps, good luck