How-To Geek

when i came back from an errand i found everything on my computer is now verticle! i re-started; i logged off and on and it is still verticle! when the computer initially log off, my prompts like 'monitor is going to sleep' are in the right direction. my monitor prompts are in the right direction. but as soon as the computer gets to were i have to enter my password to log in that is verticle on the screen as is everything else. any ideas as to how/why this happened? i wasn't home to see what happened so i haven't a clue. i've never seen or heard anything like this. i am not a Geek at all, just an ordinary everyday user. can anybody help me or do i need to bring it in for repair (expensive) work. it's only 3 yrs old. i use Vista and the computer is 32 pavillion. never had any problems til recently - thanks.
UPDATE: I figured out what to do. I went into advanced monitor settings and checked landscape. BUT can somebody be so kind as to tell me how the heck this happened 'on its own' inthe first place? As I said, it was fine, I went out and came home and it was verticle.....