Just imagine that I was really stupid and I happened to move into check and then a couple of moves went by before my opponent actually noticed ...

Which is the corrent option:

a) No-contest - both players declared nimrods

b) I lose for performing an illegal move

c) Go back to the move prior to me moving into check and allow me to move again

Remember that this tale is fictional, I need an answer to merely satisfy my curiosity!!!

If it happens in a serious, normal tournament, c. If it happens in a serious blitz tournament, b. Any other kind of games, probably a.

In blitz, you lose the game if you are still in check. If some moves have since been played and you're not in check anymore, the move stands.

In standard chess, the position before the illegal move is restored, the clocks are restored to the best judgement of the arbiter, your opponent gets 2 minutes extra time, and the game continues. The touch move rule counts, so if you have another move with your king, you have to play that. The third time you do this in a single game, you lose.

thanks guys!  very helpful!

not really sure, it happened once with me, we went back to move before error.