Instead of uninstalling and reinstalling one should try to recreate the catalogue. The full description is here:
Reinstalling any of the Adobe products will not solve anything because all the preferences and registry entries remain intact on a Windows system and I believe it is the same on a Mac system. The surest way is to use Adobe Cleaner but this tool is so powerful that it removes all Adobe products and this is not recommended to anyone here.
I hope you will try recreating the catalog to see if it resolves itself.
Prior to the error message, do you get a message along the lines of "Opening Catalog xxxxxxxxx.jpg.lrcat"?
If so, this is a common problem (fixed in LR4) which relates to a user error when trying to create a new catalog, and is best solved by resetting the preferences file. You can find instructions here:
Many thanks, I was able to isolate the offending file but wish I'd had your directions first. Your time and comments very much appreciated!
Sent from my iPad