Sometimes, we Dungeon Masters hit a slump. I know I do. It's one of those things where it just doesn't feel the same way. It's a hard thing to describe to someone who hasn't felt it. That spark of inspiration just isn't there the same way compared to earlier in the campaign. That feeling like you wish you could run a session right this moment isn't there either. Getting past it is hard but I hope that some of what has worked for me will help at least someone out there. I'll be focusing on home games as usual. Organized play is a different situation.
There are certain responsibilities that come with being a Dungeon Master. Even if we aren't feeling completely up to it, we need to run sessions for our players. That said, there is a certain contract between players and their Dungeon Master. If your off feeling is something just happened and you can't quite tell why, it's not much of a problem. For me, it would often go away quite quickly as I got back into the game after a few other sessions. However, if it comes from dysfunctional players or something else that is a long term factor, it's a different situation. In that case, you know what the cause is and it should be dealt with. When it's your slump or just general feeling, the cause isn't as blatant.
Dungeon Masters are people too. It could be that something else came up. Maybe you really got into a book series, TV series, movie, video game, whatever it is. Work and other aspects of life could also make you less enthusiastic about your campaign. For me, I find that getting some preparation done or even reading through a game book, particularly Monster Manual and Dungeon Master's Guide like books, greatly helps my enthusiasm. Finding an adventure and taking some parts you find really cool can also help, since it could replace that slump feeling with a desire to run that new cool thing. It's basically a search for inspiration at that point. Of course, getting into something else can also have the opposite effect since it can jump start inspiration. Sometimes it's worth reflecting on why you got into that other thing and seeing what you can try to bring back with you. It can lead to surprising results.
Taking a Break
Sometimes, you might just need to take a break. This is a hard thing to say, because I personally don't like taking a break in the middle of a campaign. It's how I've seen many campaigns fall apart before reaching the end. However, you can take some time between running campaigns to play in someone else's game instead. Switching sides of the screen can help with that feeling. It also depends on the kind of campaigns you typically run. If it's more episodic in nature, you might be able to get someone else to guest DM for a little while. There are some things, such as family or job related stuff, that you can't ignore either. If you do decide to take a break, finding the willpower to come back may be an issue. Be prepared for that.
The frequency of sessions plays a big part in all this. You can reasonably allow yourself a week or so break if your sessions are every 2 weeks. However, it's tougher to fit in for weekly sessions or if you run multiple campaigns weekly like a Dungeon Master super star (it takes serious dedication and is appreciated). It's not uncommon to have a bit of a break between campaigns too. The start of a campaign tends to involve a lot of Dungeon Master planning and thinking, even if it's just running a published adventure. At that time, you can reasonably allow yourself a bit of a break. It really shouldn't be a chore though. I find starting can sometimes be the hardest part if I'm into something else at the same time. Once I start, I enjoy it and might continue thinking about it for some time. I recommend coming up with a self imposed deadline just to keep on track.
When Does It Happen to Me?
For me, I can get my attention shifted to something else for a bit. Usually I lose myself in a book or a book series. Pulling yourself out of that can be difficult but I usually come back with something that can inspire me. The key, I think, is to try and not letting it affect your players. Even if you are feeling slightly off, you should still be enjoying yourself when you get there. Like all things, there are layers of severity. More serious situations might need more serious and drastic solutions.