Has it ever happened that a club went on to win the European cup but got relegated in the domestic league?

Answer 1

frank, the closest i can name you is nottingham forest. according to wikipedia:
Forest are also the only team to have won the European Cup and then been relegated to the third tier of domestic football.

although i can assure you that this did not happen in the same season.
http://en.wikipedia.org/wiki/Nottingham_Forest_F.C.

Answer 2

I don't know dawg. I can tell that there has been an incident were a team won the champions league and then had to play qualifying rounds the next season to make it to the group stages...