Halo 3: lost all progress in solo campaign

[PwnedDonk]

I was an idiot by playing nearly all the way through on the 3rd difficulty - thought I could play a random earlier multi-player campaign mission in multi-player. Despite the warning I lose all my campaign data, but that's me being stupid and not reading the warning correctly.

[Wondercall]

Soooo my bud and I beat the game but it never saved. We come back tonight starting on the last level and beat it again, still no save. What am I doing wrong?