My Canon 550ex wouldn't switch on yesterday. Completely dead. Changed the batteries (twice). Still nothing, no indication of life. It was fine last time I used it about a week ago.
Is there anything that can be done?
After trying as few things and playing around with the flash with no success, I found the following thread on the web.
The advice given is :
I actually works! To rub the terminals inside the battery compartment I wrapped a thin, slightly damp rag around the eraser end of a pencil and gave it a few spins. The terminals didn't look corroded or damaged. Afterwards the speedlite switched on, good as new.
Another solution I saw (can't find the original link anymore), was to take the batteries out and switch on and off again. Apparently this 'resets' the electronics.
I don't know if this actually works. I'm just adding it here as something to try for people who search this question.