If the 400E is anything like my 1990 300e then the sunroof manual crank is in the trunk, on the drivers side, near the shock tower, and their should be a manual crank in with the tool set. Exact location is in the owners manual if you have it.

I had a similar thing happen a couple of years ago, and it occured after disconnecting the battery for a stereo install. It might be a convience relay, I went out and bought a new part/relay, and before I got it installed, everything started working fine and has ever since. But check all your fuses as well.