I'm not clear whether your actual photo images (RAW or JPEG etc) have been trashed, or whether it is only your catalog that seems to have been lost.
On the other hand you did say the originals are all backed up so that can't be a bad thing! In the very worst circumstance you could create a whole new catalog and start over. But don't do that yet.
The challenge, as far as your editing work is concerned, is that the record of your edits is all contained in the LR library catalog (unless you were saving .xmp sidecar files back to the image folder)
However, the good news is that LR by default is set up to backup the catalog at regular intervals. By default, under MacOS, the backups are stored in a folder called 'Backups' inside the 'Lightroom' folder inside 'Pictures' for your user name (~/Pictures/Lightroom/Backups). Each backup is a copy of the catalog that's put in a folder that looks something like "2012-02-14 0838" - the first bit is the date of the backup of course, the second number a unique identifier.
With any luck* you will have a recent backup. If you double click on the most recent backup catalog, LR will open right back at the point you were at, collections and all.
It is probably very good practice to copy your backups folder to a safe external drive too in case of hard drive failure.
FWIW Ithink your 'repair shop' has gone way beyond its mandate by throwing away stuff off your computer, especially as your description doesn't make it sound like there was a major problem.
Hope you find the backup catalogs.
* Strictly speaking, you shouldn't need luck, because a decent backup strategy is important. As the saying goes, "you never need a backup until you need a backup". You need a backup!