Exit Shotwell
Locate the data folder according to your version:
Shotwell 0.12.3 and earlier -> ~/.shotwell
Shotwell 0.13.0 or later -> ~/.local/share/shotwell
Rename the data folder into something else (eg: data.old)
Restart Shotwell & it will rebuild all database from scratch now. Enjoy :)
Upvote & resteem if it helps you. Cheers