1. Pop in new batteries.
Oh and try another mouse.
Remember there is a PRANK that some do this to folk so read http://superuser.com/questions/406502/how-can-i-reverse-mouse-movement-x-y-axis-system-wide-win-7-x64
Sorry but as pranks are not always the same, how to undo is not as well.
Today I was doing normal activities in my laptop until my mouse started acting weird. It's a wireless mouse. So the thing is that it started moving the other way around: when I move the mouse left-right then the cursor moves up-down. When I move the mouse up-down then the cursor moves left-right. It's a strange problem, but if anyone can help, would be awesome! Thanks!