I loaded up a .PNG into PS. As I readied myself for the task, I accidentally pressed the 'F' key. I pushed it several times until the program was back to normal. Except now, I cannot draw, fill, erase...nada. All I have is the prohibition symbol (circle with a diagonal line through it). I tried resetting the tools, the Essentials workspace, etc.
What's the magic key to fixing this?
F is just for display and toggles through different states of display.
As for the inability to edit the picture, I would look to the layers palette. The layer may be locked or it may be a smart layer or similar.
A screenshot would be helpful, for us to see, what state your program is in.
When all else fails, reset the preferences.
"circle with a diagonal line through it"
This means you can not edit or use active tool. If you click on canvas-image what error message appears? There can be numerous reasons that you can not use some tools including that none of the layers is selected in the Layers panel or because you are in Indexed color mode, or perhaps as mentioned you are trying to edith directly Smart Object layer. Screenshot of entire window with Layers panel visible should help us to understand problem.
When I turned the machine on this a.m. the problem had solved itself. I think I saw some little blue Smurfs lurking nearby,