1. Quit Lightroom.
2. In Explorer or Finder, locate your Lightroom catalog. Typically catalogs are in /Pictures/Lightroom on Mac and My Pictures\Lightroom on Windows.
3. Find the folder whose name ends with "Previews.lrdata" and delete it. (note, you may also have a folder whose name ends with "Smart Previews.lrdata." You shouldn't need to delete that one.)
Once the preview folder is deleted, restart Lightroom. You may see gray thumbnails in the grid for a few moments as Lightroom starts rebuilding your preview cache.
Go to the folder in which your catalog file (with the extension *.lrcat) and its accompanying Previews folder (whose name ends with .lrdata) By default they are in C:/..../My Pictures/Lightroom, unless you have changed that. Move the lrdata folder to your desktop. Launch LR and it will create a new lrdata folder. To save LR the task of rebuilding all the previews, close LR and copy/paste the folder from the desktop on top of the new lrdata folder. Generally this is enough to solve the problem, but if it doesn't, let LR build new previews.
Thank you for rescuing me. I was able to find the folder (which I keep on a seperate hard drive along with my photos, all backed up again elsewhere). I deleted the "Previews.irdata" folder and another that was a "Repair previews.irdata" Then I restarted lightroom by double clicking on the latest catalogue file. This opened lightroom with the catalogue I had been using when it went pear shaped. I was trying to load old photos into lightroom and I guess I had far too many at once, several hundred, and my computor could not cope with the work load.
Thank you again John for your promt advice.