Was your library on a removable (USB) drive ? The letter may have changed. Calibre would revert to a default location (on C
Search all drives for
Metadata.db
FWIW The small (file size)one is the current empty library.
Note the location (path) to any others.
Right-click the library Icon: create/switch:
use existing: <navigate to that place you found>
Did you win?
Repeat for any others (usually only needed folk who have multiple Libraries)
NOTE: Use of libraries on NAS, Clouds could lead to corruption. Suspend sync while using Calibre to make ANY changes.