Hi Mafibacon
I use a Dell screen (with a desktop pc though) at work and this 'screen flip' is a built-in feature on it - I expect that your Inspiron screen has the same feature. To change the setting, use CTRL+ALT+arrow (the arrow to use is the one pointing to where you want the top of the screen to end up, so 'up' arrow for the normal, landscape view.
I expect that you hit the key combo by accident while typing (I have done this several times). Hope this helps...
Gadget 