I started a begginer campaign, my first, two days ago and have played every turn. When I logged on for the next turn, I was no longer in the campaign. No enemy units were near my castle. I didn't lose, I was just gone. What happened?

Probably everyone else in the game went inactive and you were the sole survivor. The game ended and you won! Not much fun to win that way though.

Which game was it? You can look it up on the scoreboard and see if this is the case.
Cheers,
H.