I have Photoshop CS5 extended and a Mac with OSX 10.6.8. I was altering the 1st and only layer in my .psd file by filling areas with the "content aware" setting. (The background is blank.) It was working great, too! Then suddenly the "fill" selection under "edit" was greyed out, as well as most of the other things on the edit menu. It doesn't matter what I try to select in that layer.
I shut down Photoshop and restarted my computer, but it didn't help. Duplicating the layer didn't help, either. I CAN fill and stroke in the background layer.
A screenshot showing your entire Photoshop session might help; someone might be able to spot something wrong.
Note that a lot of stuff in the Edit menu is normally grayed-out, if you've just opened a document, are sitting on the background layer, and nothing is selected.