Unable to produce frame when it worked moments before?

This is kind of ubsurd.
rendering two scenes via Media Encoder, the first renders perfectly, the second then fails when I foolishly tab to another window briefly.
Now it won't even start rendering, and just kicks up a "Unable to Produce Frame" Error.
Tried rendering the first scene again, and now that does the same! after rendering perfectly moments before.

I've tried updating graphics driver, and to render with and without hardware acceleration, but now everything rendered from AE through Media Encoder kicks up that same error.
Why on earth might this be? Did my GPU become obsolete in the 10 minutes between renders? did tabing to another window perminantly upset Media Encoders sensibilities? Any thoughts on how I might approach solving this greatly appreciated!