Life happens sometimes and it sucks. It can happen in a variety of different ways, but whenever it happened it always seems to come gunning for whatever it is you enjoy or lets you relax the most. Sometimes it takes away time, sometime it takes away energy, and sometimes it takes away players. Thing is, as a GM, odds are at some point you'll come across a time when a game has to go on hiatus. The question is, what does that actually mean?
For the purposes of this I'm defining hiatus as any time a game takes an extended break with the idea being that the game will continue later. That's it. It really is that simple.
Be Prepared For It To Never Return
Seriously, whenever a game is going on an extended leave you have to understand that there is a chance it won't come back. Time is fleeting. People will end up with other things on their schedule. If you replace the game with another game it is entirely possible that the other game will become the new 'thing' as people become more and more invested in it. This isn't to say it is definitely going to happen, but be prepared for it to happen.
A Second GM Helps
If you are replacing your game with a different one then swapping out GMs can help to both make the game feel different and to ensure that the original game does come back. By swapping to a new GM you have a whole different feel to the game. That "new" game, complete with new GM flavor, can then be set in everyone's head to be a shorter game or one that fills the gaps of the original game. From there the two GMs can work out when one game will end and the other will take back over, or if they want to go into more of a time share situation.
Making It Linger Makes Everyone Suffer
I know I said it might mean the end of your game, and you don't want that to happen, but you also have to realize that trying to force a game when it can't happen is also problematic. Running when you can't reliably have players isn't fair to anyone. GMing when your stress level is through the roof is only going to cause problems and create tension and resentment in your game. The thing is, if you think a game needs to go on pause - even if it means the death of the game - you are better off just biting the bullet and doing that then trying to drag it out. Clean breaks are sometimes best.
Communicate With Your Players
Finally, while full reasons don't need to be disclosed, you want to communicate with your players about it. Obviously you need to let them know the hiatus is happening but it is also good to let them know when you think/hope it will come to an end. As you get closer to the time stay in contact. Trust me, it's a lot easier to revive a game when it doesn't come so suddenly to everyone.
If It Dies..
I know I said finally above, but this also bears saying. If the game dies, let it go. Maybe you can start a new one up when folks have time. Or, who knows, maybe you'll be able to revive it later on down the line.