I downloaded a new version of Lightroom 5.7 because a Windows upgrade problem wiped out my prior version of Lightroom 5 along with other programs. Now I get this message: "Lightroom cannot use the catalog named 'Lightroom Catalog 5' because it is not writable and cannot be opened. This could be caused ... because another Lightroom application is using the catalog..." How can I get my new version to use this catalog?
Two possibilities for you to check. Using File Explorer, open the folder that contains your catalog. Look for a file with the same name as your catalog, but with the lock extension. This file is generated when Lightroom opens, and is normally deleted when Lightroom closes. If Lightroom doesn't close normally, that file occasionally gets left behind. If there is no lock file, then check the properties of the folder containing your catalog and make sure it isn't marked read-only.
I did not find a *.lock file, other than when it does appear in Windows Explorer when the LR error message window (cited above) opens. I changed the folder to remove the read-only designation, but that didn't help either.