In the same folder as your catalog file is a file whose name ends with .LOCK
Delete this .LOCK file
Do not delete anything else.
Thanks for the very prompt response! I'd forgotten about the .lock file.
I deleted the .lock file, then attempted to relaunch Lightroom by double-clicking on the desired catalog file. Lightroom launched with the same "catalog not found" dialog. I selected the desired catalog, clicked the "check integrity" box, and tried to open it. After about two seconds, Lightroom hung again. When I force quit it, the Adobe Lightroom application was not running.
However, the external hard drive on which the catalog and images reside could not be ejected (application in use).
Any other ideas? Thanks!
Problem solved. The catalog was corrupt.
Moral of the story: always keep a backup (or two) of the original RAW images.