Restore the photos from your backups.
If you don't have backups (why not?), then you need to make backups in the future.
You can recover small size, low quality previews previews from the Lightroom preview cache, using a plug-in like this one: Jeffrey Friedl's Blog » Jeffrey’s “Extract Cached Image Previews” Lightroom Plugin
Your pictures reside on the hard drive just like every other file, not actually "in" the catalog. Check your "trash can" or "recycle bin" to see if the delete can be undone.
If your pictures are not on a backup device or on an old memory card, they are gone if you don't have a backup.