Check if your save files exist by going to the save folder:
Save folder locations:
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)
If they exist, try the following:
1. Make backups of your save files
2. Open up the game and make save files in all the slots your original save files are supposed to be in.
3. Copy your actual save files into the save file folder, overwriting the save files you made in step 2.