When I hit something like this in my comics I walk away from it and do something else. I go and do some drudgework on another strip I haven't finished yet - tweaking all the word bubbles so they're not perfect ovals for instance - a tip for which I curse Dave Gibbons every time I do the tedious plod of a job because I got the tip from a book he wrote. (But dammit he's right a page full of perfect oval speech bubbles DOES look like crap!)
Anyway.... Just go and do something else and forget about it. Some part of your brain will still niggle away at it and come up with a solution.
For a while, when I got totally unproductive and the ideas weren't there, I used to start strip after strip in my notebook. Do one or two panels with no plot in mind. Sometimes, out of nowhere and sometimes weeks later, while doing something totally unrelated - doing the weekly shopping or hoovering the stairs - one of the half started strips would just appear in my head with the rest of it just there, punchline and all, just impatiently waiting for me to write it down. Deep down inside my brain some bunch of neurons had been beavering away trying to come up with the answer to the problem I had set them.
