I'm not sure why it might have happened, but there is an option to change the rotation in the Control Panel-Display-, then select the "Change Display Settings" link on the left side. Once there, use the drop-down menu in the "Orientation" section to set it as you'd like..
Hope this helps.
Grif
This morning my windows 7 PC displayed the message, " updates need to be installed, then your machine will shut down and restart." This looked to me like a Windows update, so I did the update. The PC shut down but, contrary to what I expected, it did not restart. So I restarted it myself. When it booted up, everything on the screen was upside down! And my mouse cursors were totally reversed! An impossible situation! I clumsily located my anti-virus program (avg).By standing on my head I was able to read the scan result, which said "No Threats Found" NEXT I re-re booted-----to no avail. Then I stood my monitor on it's head to better perform a "restore" This corrected the problem! The reason I bring all this up is curiosity------has anybody else experienced this nutty thing, either this morning or earlier this week? Am I the only on?. Was this the fault of the "update"? or was it a sneaky virus that hitched a ride on MS update? Anyone?

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