With your images on the backup drive, you just need to COPY them back to the original location on your main drive.
Do not try to import them again. The file references are already "in" the catalog. You just need to put the images back where LR was told they were originally at.
How do I know where the original location is? I think I deleted the original folder... should I just create another one with the same date?
In the grid view, right-click on a "missing" picture, select "go to folder in library".
If the Folder view does not look like what Finder shows you, right-click on a folder and select "show parent"'; repeat as needed.
This will show you where LR thinks the file originally was.
You can either try to put the images back where they were, or put them back somewhere else and "relocate" the folder in LR.