Open a recent catalog backup and see if the keywords are in there. Keep going back in time, opening earlier catalog files, until you find the keywords.
Thank you VERY much for your prompt reply. I downloaded LRcat backups until I found the problem, or what I suspect was the problem.
I bought a new iMac earlier this year and transferred my data across. To make a long story short, over the space of 1 week, the Mac had 60 kernel panics (kept stopping and starting) and corrupted my Drobo backup. When presented with the problem computer, Apple issued me with a new computer immediately and I was given the contact details of 2 senior technicians here and in Singapore to contact to try and recover my lost/misplaced/corrupted data. It has taken many, many hours but things seemed normal until my discovery of the lost keywords.
I fear they are now lost forever and I will have to re-enter them. I do have the metadata which gives me a good starting point and I can be very dedicated and patient when I am compelled to, as evidenced when reconstructing things with the Apple technicians. It took weeks of work. Apparently no-one else has ever has 60 kernel panics (or so they say, I am not at all sure what they are exactly) before. At one stage, all was lost, but we managed to get the photos back and it certainly tested even the most senior people out.
If you can think of anything else I would appreciate your input and once again, thank you so much for your help. I have been on a very large learning curve!