1) Your video card supports screen rotation, and it probably has
a hot key set up to do it. You pressed the hot key by accident.
Bring up the driver for your video card.....right click on the
desktop, bring up the "Properties". Go to "Settings" tab, then
click "Advanced". That will bring up the custom control panel
for the video card. You will find the rotation settings somewhere.
2) Press the "menu" button on your monitor, and see if you can fix it
there, a very easy thing to do and fix.
3) Press CTRL/ALT/right arrow one after the other (not simultaneously) and it will switch the direction of your screen.