I see that this is your first post, so I am assuming that you have properly identified the program as Lacerte and that you prepared the LLC in the Partnership module. I doubt it really disappeared, but even if it did not - this is a clear reminder to BACKUP, BACKUP, BACKUP.

My best guess is that you accidently clicked one of the Views along the left column and that removed your return from that view. Click All Clients and HOPEFULLY all will be good with the world.