Check the obvious first!

Time to photo cutting the birthday cake so I attached the flashgun and - no flash. Checked settings, no difference. The mode on the flash was TTL, not P-TTL unless on wireless mode, in which it would work off-camera. The camera was showing a GPS fault setting so it assumed a GPS unit was attached and not the flashgun. Checked it was locked on ok. The clues are all there but after half an hour of poring through the manuals to no avail I wiped the contacts and, hey presto, it works!