"collections" in library are missing

Greetings. I have an issue that may be routine to many of you, but has left me flummoxed. Somehow,  during a data migration to my current computer (an Imac m3), something happened.  My lightroom catalog tranferred over, and everything looks good, but all of my collections are gone. I had a good number of edited photos in there and right now it seems like I'm going to have to start over. I'm pretty sure (haven't checked) that the collections should be backed up on an external hard drive. Is there a way to move the Collections folder from the external hard drive to my computer? Will it just appear in its proper place in Lighroom? Or do I have to start over sorting and developing photos? If anyone has an answer, I would be thankful. Please explain the steps in simple terms if possible. If you get too technical, I'll just be sitting here open-mouthed and confused. Thank you in advance. I'll check for a response in a few days. My email is altifrons@aol.com.