It’s horrible. They’re like mosquitoes trapped in amber. (1×03, “The Ghost Network”)
Strangely enough this is also how I feel at the end of a long week when I can’t focus. I’m sure you know the feeling. There are ideas but then it’s time to get them down on paper/tablet and… this is your brain on amber. Nothing moves.
The answer seems to be, just draw something unfocused if you have to. It’s probably not what you really want, but just get something down (draw more than you need), zero in on different details, figure out which specific part of it you really want to work on — and then start carving it out of the amber, even if it’s still stuck in there for today.