Hi wollster,
1) Try this first: CTRL+ALT+Down or CTRL+ALT+UP. Because the display is mirrored you might need to press the opposite of the normal key command.
2) If that doesn't work, try:
Open Control Panel -> Screen Resolution -> Orientation. Use the drop-down menu to change your orientation back to upright.
3) Some people have reported that turning the PC off and unplugging the monitors for 5 minutes reset this. If the above two solutions don't work try that next.
It sounds like this is probably more of a graphics card issue than OS issue, so you might also be able to go into your graphic card software and reset it from there.