First, I'd like to understand why that happened. The missing collection was only on my iPhone and I thought it would just kind of stay there no matter what catalog I was using on my LR Desktop. Then I'd like to know if I can restore that old collection. Thanks in advance!
Have you been connected with a different catalog as you used to have synced to the Lr Mobile app.
You can only have one catalog connected to your account and when you switch to a different catalog normally Lr Desktop asks you if you want to sync against this catalog. Can you remember your steps?