Speedlite 420EX lost a plastic part

What are the issues your having? Without an idea of what the photos look like and what settings you are using there's not much help we can offer.

yes it is compatible

This may seem dumb to ask but have you made sure all batteries are fresh (even new ones can be bad), they\'re inserted correctly observing polarity and the contacts are clean in the flash? (You can clean them using a clean, dry pencil eraser and a Q-Tip dipped lightly in alcohol.)



Also, try mounting the flash on your camera, turn both on and reset your camera to factory default settings through your camera menu settings.

The buzz noise may be the zoom motor in the flash activating as it starts up. If it encounters resistance it will shut down to protect the motor and flash. It's impossible to tell if something has jammed the zoom motor or path without opening the flash (I suggest you don't). If mounting the flash on your camera, turning both on and resetting your camera to factory default settings through your camera menu settings doesn't reset it you'll probably need to send it in for repair.

You can get the user manual for this device from this link

http://www.scribd.com/Speedlite-420EX-Manual/d/11516507

You can download a manual from http://www.usa.canon.com/consumer/controller?act=ModelInfoAct&tabact=DownloadDetailTabAct&fcategoryid=316&modelid=7394

Please click the link below to download the manual:
Canon Speedlight 420EX Manual

Send the flash back to canon for Service this is not something that can be fixed in this forum. Do not use the flash the burning smell indicated a potentially dangerous problem and fire hazard.

Part # CY2-4165, order from Canon. Four screws and a plug in cable and that is it.