Thank you very much! I asked my hosting provider to restart the MySQL server because I can’t access the shell. They were unable to do so because of an unrelated technical problem but when they did, all the categories and tags reappeared. I am immensely grateful I don’t have to re-enter them on 327 posts and pages!!
I’m also glad to record here the solution was as simple as restarting the SQL server. The three links which Mark passed on are invaluable.