Man, I am beat. These are the days I kick my own ass for not having a bigger backlog of articles on hand. I'm sufficiently fried that it's hard to pull together any kind of coherent thought. But it could be worse.
I could have a game to run.
There is no feeling I hate more than coming up on a game I need to run when I'm this drained. You scramble and try to pull it together, but you know you're just not going to be able to really put your back in it, and it's probably going to suck as a result. Still, you gotta do something.
Everyone's got their own tricks for dealing with this situations. Some people like to turn to pre-published adventures in these situations, but I've never quite gotten my head around that. If I'm too tired to plan, I'm definitely not going to have the energy to read and capture the intent of someone else's adventure, to say nothing of finding a way to tie it to my game.
I admit I'm not above proposing a different game for the evening, but assuming I do need to go forward, I reach deep into the bag of tricks.
First option is one that I am eternally grateful to Robin Laws for laying out in Feng Shui. Pick 3 interesting fight scene locations, then just come up with enough of an excuse to thread them together. Fight scenes can be used pretty flexibly in this regard to cover any kind of set piece, like an investigation or a party. The trick is that each scene needs a gimmick, just one, to make it something more than generic. A fight scene needs to be in an interesting location, a party might be a masquerade, an investigation might be on the clock. If the gimmick pops, the scene tend to take care of themselves as players bounce off the unique element and make their own fun.
If that seems like too much work, or if the game is at a point where that's not much of an option, my next favorite cheat is to give the players a goal and resources but a muddy path to doing it. This is kind of shameless, but it means a large portion of the session will be occupied by player driven action as they plan and prepare. You don't want to do it too often, and you need to make sure the payoff is worth it  but it can be a fun distraction, and since your job is mostly to answer questions, it's a lot less work.
If I'm really desperate, I'll steal. From books, movies, TV, or even old games I've run or played. You need to be aware of what your players are familiar with, and you definitely need to be careful to not do do the same thing twice, but this can be the easiest thing of all, but only if something jumps out at you as a good match.
Lastly, I'm willing to hit the eject button if things really go south. You don't do this lightly - people have held up their end of the bargain, and you should respect that - but if the game is getting to the point where it's less fun to be playign then no, then it's time to say "Guys, I'm just not feeling it tonight. Who's up for some Dominion?". If it's going as badly as you think, then they'll be happy to take you up on it.
1 - Part of this is because adventures are horribly presented. Even really good ones tend to be total crap in this regard. Part of this is because the needs for reading are not the needs for use, but more it's because we default to the gygaxian map and key model. Most good adventures have some sort of thread running through it, but even if the author explicitly calls it out, you still end up having to piece things together from the corners of the encounters. When I make an adventure for myself, my goal is to create a situation and wrap my head around all the parts, so I can roll with things. The method of presentation for most adventures supports a sequence of events, which is all well and good, but getting the complete picture into your head is hard in the best of situations. When you're tired? No chance.
2 - Protip - end the episode on a cliffhanger, and do the payout next session when you're on your game.
3 - One other dirty trick to make any investigative game work is to just say yes. Your players will ask if things are so, just say yes. Roll with it, and the story writes itself, at least so long as they don't catch on that you're doing it.
4 - Another Protip - Stealing from movies is great because you can call it an "Homage".