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First try the following keyboard combination:
Ctrl-Alt keys with the Up/Down Arrow key.
If that doesn't fix it, go to Control Panel > Display > Settings > Advanced, and look for a tab that says rotation. Then choose the option that says landscape or standard or 0 degrees (the exact wording depends on your display adapter).
If there is no rotation setting in the Advanced options, try to locate any feature of your display properties that enabled the monitor display to be rotated - e.g. a display icon in the system tray.
Hope this helps.