Hooray! Problem solved!
Changing the screen resolution right through the range from highest to lowest had no effect at all (except that it b***d up the layout of all my program icons!) but deleting the preferences file - or moving it to the desktop and re-naming it - solved the problem.
When I re-loaded Quark all the palettes reappeared, and, what is more, when I re-loaded a document I had just finished, nothing had changed - all the parameters of the document were exactly as before !
Very many thanks for the advice - next time I have a problem I'll know who to ask!