We had a really good play-by-post game going for a while, but now it has seemed to stall. I've talked with the member who seems to be most responsible for stalling the game, and he still shows interest, but it's too low on his priority list. In the meantime, the others seem to have lost some of their interest. How can I reignite the campaign?
Here are some ideas:
Bring in a new, enthusiastic member. New blood can kickstart anything.
Get them to pledge commitments. Ask everyone (publicly) how much time they can commit to the game every week, and ask them to keep to it.
Bribe them. Offer a cool magic item or other appropriate boon to each player who meets a certain time commitment for a designated period (one month, etc.).
Dangle shiny lures. Much like bribery, you can send out campaign story teasers, have NPCs "taunt" them IC, and so on to spur them to come back and play. Remind them of unsolved mysteries, rumors of treasures to plunder, or whatever makes sense.
Don't force them into it. If you do that they'll have even less interest than they did when they bailed the first time. Offer to start it up and invite everyone, but make sure there's no pressure to join. Run it for the players that still have interest and replace the losses with new players.
I say this because I'm of the opinion that some PCs are lost causes. If you spend too much effort on them, you'll alienate the PCs who did have an interest. It's great if you can win over the uninterested players, but they're only worth so much effort.
It's important to note whether or not the campaign has stalled as the result of life issues or disinterest with the campaign itself. These can lead to completely different suggestions. But here are a few generic suggestions:
Campaign reboot with new characters: As has been suggested, start over at a point further ahead in time, with as little or as much changed in the campaign world as you wish, but with new characters. Make the new characters children of the original characters, or tell the players the characters disappeared many years ago and their new characters are dedicated to finding out what "really" happened to the original group.
Mystery: Everyone loves a mystery. In your next game start with an unusual or bizarre happening (perhaps the sky has a new moon at night, or earthquakes start hitting the large cities, or all the trees in the world start dying). This can lead to a connected adventure or perhaps is just part of some overall world change that players can only react to and not stop themselves. If the mystery is more personal (say, everyone at the local inn the players' frequent is killed and the inn burned to the ground) it should motivate the characters directly to find out what is going on, spurring reknewed interest in the game.
Bad Guy: Let's face it, a villain or group of baddies can inspire players simply because the party wants to put them down or get even. Perhaps a fleet of powerful pirates has suddenly begun attacking the coast cities of your campaign world, raping and pillaging with shocking atrocities. Maybe a prophecy has been fulfilled and a demon king is now free to roam the earth with his followers. A long banned evil religion suddenly has a powerful and charismatic leader who is quickly gathering followers and has destroyed several temples one of the PC character clerics worships. Half the appeal of Star Wars was Darth Vader; a good villain or opponent can fuel lots of great sessions.
Start the next adventure "in the middle". If you have an intriguing enough idea, it should pique the interest of players enough to wonder "what happens next", or even "how the heck did we get here???". Some suggestions: The characters awaken with no belongings, chained in cavern far below the earth, watched over by a troll guard, with no idea how they got there; Characters are in the middle of a siege of a city, surrounded by an army of orcs; Characters start out shipwrecked on a tropical isle, with all their belongings but no way home; Characters slowly come out of a fog; they have been dominated by an ancient artifact and the magic has worn off, but for the last few months they have been doing the bidding of an evil mage and are now on the wrong side of the law; the characters start in the middle of a long caravan journey to a distant realm that they have been hired to guard, being attacked by bandits. Be warned, a lot of players hate this sort of thing because they feel they are being "railroaded"; however, if you can get them to go along you can have a lot of fun with it.
One way is to move the timeline of the game forward to the cusp of something amazing happening, so the change of vibe at the table is mirrored by a change in the game itself.
Change the world, kick it in the teeth in a way that gives the PC's and NPC's entirely new contexts but still leaves them with plenty to do.