You know the feeling, I'm sure. You've got game in two or three days. You've got players telling you they're looking forward to the next session, or e-mailing to make sure things are still on. And you've got a blank piece of paper with a little note at the top that says "Session Prep" with the coming game day's date right beside it. You've got nothing, but you need more than that. So what do you do?
Step 1: Don't Panic And Don't Blame Yourself
Block happens. I don't want to call GM prep block writer's block, but it's pretty close to the same thing. It can be incredibly frustrating, and it will only get worse if you panic or start blaming yourself for not being more ready. Instead you need to try and relax - even if this means doing something else for a bit. Go for a walk. Play a videogame. Start writing a novel. Do something that puts you at ease.
Step 2: Review Your Notes For The Last Session(s)
Refreshing your memory of what happened in the last several sessions will often kickstart your brain for going forward. You've got storytelling muscles in your brain, or you wouldn't be a GM at all. So feed those muscles something to work with. Try to remember details that are connected to what is in your notes but not recorded. Did you leave any loose ends? Was there a clear path the party was going for? Good. Work with that.
Step 3: Discuss With (Some Of) Your Players
Talk about the last session with your players - or some of them at the least. Flat out ask what the player thinks is happening and what they'll run into. What are they looking forward to? What do they want to do? Do they have any goals or ideas you should know about ahead of time to make sure you're ready in game?
Even if you never figure out what you were going to do with the coming session originally, you'd be amazed how much content you can make just trying to help your players achieve their little personal goals for the game. A shopping session with a GM just expecting to go through tables is boring. A shopping session with a GM whose had time to prepare some stuff for specific items can be a real blast.
Step 4: Add Combat
You've reviewed your notes, you've talked to your players, and you're still lost. Let's try winging things now. Are you in a place where you can add combat? If not, why not? What has to happen to get the PCs somewhere where combat can happen?
Don't get me wrong, every session shouldn't be full of combat. But if you're in a pinch and need to stall things out, some combat can be just what is needed to fill time between player musings and other RP. And by combat I mean any mechanically intensive aspect of the game, be that baking cakes, having arguments, chase scenes down a mountain, or - y'know - combat.
Step 5: Take A Sharp Left
Ok, combat is out too. Maybe it's time to take a left turn. Add an unknown facet to the story not there before. Were the PCs helping a prince try to relcaim a throne from his brother who usurped it? Why not add a third faction that just shows up with guns and ships and all sorts of crazy stuff?
Alternatively, consider the PCs situation. What's the worst thing that could happen in that story right now? Make that happen. Don't worry about how. Just make it happen. You can fix it back into the story later.
Step 6: Do Something Fun
Just do something fun. Pull the PCs into a dream story. Do something new or weird. Have fun. The wackier the better. Who knows, while planning it out you may just get your brain going again on the original idea.