This was a weird one!  On one of our site collections for no particular reason the Quick Launch menu disappeared.  We’re still not sure what happened, but here’s how we fixed it.  I can’t take credit for this as my co-worker, Aldo, figured it out.

After a bit of research, he determined it was limited to a specific master page.  Once I opened the master page and made a small change and saved it, the Quick Launch reappeared. 

If you read this and have an idea on the cause, please comment.