That happened to me once (of course on a tight deadline), and for the life of me I could not figure out what the problem was. I actually created a support ticket and they couldn't figure it out either. I now think that the system was probably hung up in the middle of a sync.
Ultimately I "unstuck" things by adding myself as a lead to the missing campaigns within SFDC, and then going back into Marketo and syncing the lead. I'm not sure if that's really what fixed the problem, but my campaigns were suddently visible immediately after.