It does sound like some system issue - either corrupt file system, lack of disk space, etc.
Try renaming the library folder(s) so that AC cannot find them based on the path(s) stored in your PLN. Will the project open then (with lots of missing objects of course)? If so, add one library at a time in Lib Mgr to track things down.
When you say it will not open PLA's either, which have all lib parts embedded, that makes it seem like something else amiss on your system. Did you do a full disk check with the repair option? (Requires restarting your Windows machine to do the repair on the boot drive.) Is there ample free disk space?
Do you have a ton of temporary files that Windows has not cleaned out? In Windows Explorer, display the path %temp% and %tmp% and get rid of stuff...
One of the forum moderators
AC 28 USA and earlier • macOS Sequoia 15.2, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB