I've managed to import from a very old backup and have re-catalogged about 2,000 images. I have no idea what went wrong, but this is a frustrating bug.
From memory- The "Task Manager" syndrome was often accompanied by a "Locked" catalog- the cause being a catalog that was not closed correctly.
Do you know where your "Main" catalog was located? (In your 'Pictures' folder perhaps?)
Find the "Main" catalog xxxxx.LRCAT,
If there is a file that ends with the file-type suffix of .LOCK, then To "Un-Lock" the catalog: Delete that single file, and ONLY the file xxxxxx.LOCK
Now try to open your "Main" catalog by double-clicking on the xxxxx.LRCAT file.
If it opens you are 'back in business' with that catalog.
It sounds as though you have recovered from a Backup catalog and continued to use this.