Well I have a Canon G3 and I have been useing a Canon Speedlight 420EX.
It has been awsome! All my pictures have been devine.
I start taking some Christmas pictures and all of a sudden the pictures are very very underexposed. The flash is shooting full out and you can barely see the image. If I take the flash off the camera....the pictures look fine. Add the flash and nothing but dark dark pictures. Changed batteries and same thing. To me it seems like the timeing is off. Like it takes a picture after the flash is done. These are not freak settings F Stop at 3, Shutter at 1/60. The camera is on the AUTO setting. I'm clueless!

Make sure that the flash is fully seated in the hot shoe on the top of the camera, I have had problems when seating my 550EX on my G3, it is a little tight. You might find it isn't all the way on and that might be causing the problem.

Also make sure all the contacts are CLEAN - both on the camera shoe and on the flash foot. Sometimes a thin film you can't even see - like a fingerprint - is sufficient to break contact on one of the pins......and that's all it takes.