well if you lost did why where you rebuilding images in the first place? Was it because you say all your images?
The only way I know of that you can lose images in the program running a rebuild image thing is if your server path changed and the images cant be found the entries will be deleted from the database. That usually would mean you lose all your images. Correct action there would be to correct the server paths in the settings to the proper new server paths. restore a backup of the images table and you should be fine.
if you lost only certain number of images and the rest are fine I would suspect some sort of data lose issue on your files directory where the actual images are not on the server and thus same thing if you rebuild images and they cant be found they are deleted from the server. Correct action in this scenerio would be to upload a known good backup of your actual files directory and restore a copy of your images mysql table.
If you dont take site backups and database backups and have nothing to restore from then the answer is yes the images are gone.
Now of course there are stock generic advise since I have not seen your site here to really respond accordingly.