My Illustrator document has a transparent background and some green guidelines. How can I get rid of these and return to a normal document?
That is how a document created with a "Video" profile is set up.
Turn the transparency grid on or off through the View menu (View → Show/Hide Transparency Grid or SHIFT+cmd+D).
The green guides are the video safe areas and center marks, you can turn them off by enabling the Artboard Tool (SHIFT+O) and toggling the switches in the control bar:
From what I can tell (looking in Illustrator and this thread) there is no way to turn off video & title safe guides if you select a video profile for the document. How annoying is that? Video and title safe is an anachronism that hasn't been an issue since the demise of tube TVs. Please, Adobe (Apple and every other software developer), let the user decide.