Can't access to ubuntu due to a screen rotation

Okay.

I had a problem with some text resolution and I decided to check out the configurations pannels.

I went into screen resolution pannel and I checked out what "screen rotation" was.

I did a "upside down" rotation.

Until there, I saw my whole screen upside down but I could not control anything but my mouse.

I could click anywhere, I could not do anything.

Now I restarted the computer and went onto ubuntu, The login pannel is ok (not upside down)

I just login, then the damn mouse goes upside down and the begining music goes on.

Until there I only see a beige background and my upside down mouse, making me unable to do anything.

I do not see nay icons nor control pannels anymore, I just can't do anything.

If anyone knows a way to fix the upside-down screen "problem", please let me know, because Ubuntu is absolutly unuseable....