Are you playing on browser? Check that your cookies aren't being wiped automatically by the browser.
Otherwise, can you check to see if your save files are still there? They should be located here:
Windows: C:\users\(username)\AppData\Local\UserData\Default\Lust Doll Plus
Mac: \Users\(username)\Library\Application Support\Default\Lust Doll Plus
Linux: \Users\(username)\.config\KADOKAWA\RPGMV\Default\Lust Doll Plus
(May need to sudo nautilus if you don't have access to .config)