Writing's a lot like programming (I don't think this is the first time I've equated this, and if it is, it won't be the last). Sometime's you're confronted with a problem. You want a character or characters to get to a certain point. You know vaguely how they get there, but writing doesn't work in vagaries. No, you need to specifically say "They did this, then this, then this" On top of all that, you have to make it plausible, exciting, and succint. Hey, no problem, Mr. S.
Well, here's a strategy to try, if you find yourself in the same situation - work backwards. In this case I had a more tangible idea of how the scene ended rather than how it began, so I tried thinking about "Well, how did they get there in the first place, the very previous step," and then before that, and before that. It's not flawless, I'm still stuck at the very beginning, but now I've got some progress, and progress is everything in writing.