My tablet accidentally hit my keyboard, and I have no idea what keys it hit, but one of those grey right-click pop-up menus appeared for a second before the screen went black and then came back up, only flipped on it's side. What do I have to do to reverse this?

Well, you don't say what graphics you are running... ie your motherboard graohics or vid card... but rclick a blank space on your desktop, choose Graphics Properties or somesuch... else Properties, Settings, Advanced, then your graphics driver tab... and you should be in a menu like that of the first option if it existed. Rotating the screen display will be there.
Somewhere.

Is your computer a dell????

Otherwise change the rotation settings under the graphics card properties :)

please try the following key conbinations. CTRL+ALT+UP ARROW or DOWN ARROW.

Generally its under the ATI/Intel/Nvidia control panel which is often in the system tray

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