Flash failure
Clean the contacts first. If possible, get one of these:

https://www.ebay.co.uk/itm/111741724919?hash=item1a045368f7:g:ZjUAAOSwYUZcMFYu&a...
The electrical contact equivalent of a lens brush. They are excellent at cleaning contacts. Only have 1 to 2mm of bristle showing.
RS (Radio Spares) sell the same thing for a wee bit more money:
https://uk.rs-online.com/web/p/etching-accessories/0514868
Best regards
Inspiration is rarer than a plate glass camera.....
Both the *istDS and the K5 are incurably addicted to old glass
My page on Photocrowd - link
RobL
Member
Warwickshire