Crisis!

The cat has gone for one of her periodic strolls across the keyboard and managed, very impressively, to rotate the screen by 90 degrees. I've looked in all the places I consider obvious on my control box, and even tried to search in Microsoft Help, but nowhere can I find how the hell to reverse it. It must be a Microsoft thing, bacause when the computer wakes itself up the initial DOS stuff and the first Windows splash are the right way up.
I know that some of you are wise in the ways of computers (wiser than me anyway) Any suggestions?.
Apologies for any typos - Small is helping, along with the more obvious impediment.