I've just reinstalled and all of my Lightroom stuff is in one folder. So I simply pointed Lightroom to the folder once it was reinstalled but I keep getting that message saying "Lightroom cannot use the catalog because it is not writable or in use."
Any ideas what's gone wrong?
Try looking for a file whose name ends with .lock in the same folder as your catalog file. If there is such a file, delete it. Do not delete anything else.
Found, deleted, all is good, thanks!