@wdwyer:
You don't give other important information to aid in diagnosing your issue, so I will default to the following sensitive and dangerous procedure. Be careful.
[1] Open "C:\Users\wdwyer\AppData\Roaming\calibre\gui.j son" in a text editor.
[2] Save an original copy of it elsewhere.
[3] Search for: "library_usage_stats": {
[4] Carefully delete the lines of odd or problematic library paths.
Example of a line: "S:/Calibre/CalibreZotero": 1436,
[5] Check and double-check your work to ensure it is "symmetrical".
[6] Check it again, then Save.
[7] Open and save an original copy of "C:\Users\wdwyer\AppData\Roaming\calibre\global.py " in a text editor.
[8] Find your equivalent of this:
# library path
# Path to directory in which your library of books is stored
library_path = u'S:\\Calibre\\QuarantineAndScrub_CalibreMain'
[9] Change the path to a known valid library on your C: drive, or least on a local drive. No network drives. Keep it simple. Use the original format for the path shown in global.py, not what Windows would consider "proper".
[10] Start Calibre. If Calibre crashes, your work was not properly performed. Fix it and try again. You have an original backup to use to start over with.
[11] After Calibre starts properly, use the Library menu to find your good libraries and to switch to them.
DaltonST