Your profile says 10.5.8, that is Leopard and cannot upgrade to Mt.L.
Snow Leopard is 10.6, and the 10.6.8 (latest update) can upgrade directly to Mt.L.
So change you profile.
You do have TimeMachine running, of course. Then restore from TM date 12/22/12 or before.
Or you take it from other backups.