It sounds like Poser lost the library references. That's no problem. Do this: https://support.smithmicro.com/customer/portal/articles/1982174-solving-weird-issues-with-poser-by-removing-the-preferences

Deleting the pref files will force Poser to recreate factory-default prefs when it next runs. You'll just have to set your preferences back up, but this should fix the issue. If Poser is able to run, your libraries are not "gone", Poser just whacked up its somewhat dynamic prefs.

IF, however, all of your content is truly gone, after you have searched for it on your disk, then you have bigger problems than Poser getting borked up and you should immediately start backing up what you can from that hard-drive, since it's about to bite the dust. :) (Very low probability that this is actually the case, by the way.)