Several people have reported on this issue in comments on the Halloween Glitches post here. It’s happening more often and if it affects your Lisa or Homer, then you’re stuck without the use of those two characters.
It seems to happen to characters who are on goo-collecting tasks where they wander around. The kids going trick or treating, Marge vacuuming up ghosts, etc. (I’m thinking that at some point they wander too close to the edge or water and get stuck off the edge, but it’s just a guess). You can’t tap them to complete their task.
Most of the characters are associated with a building, so you can store away the building, and when you place the building back in your Springfield the character will be free. (Doing this kills any job the character was performing and will reset the earnings of the building, but that’s better than having the character off the map!) Unfortunately, the only characters this doesn’t work for are Homer and Lisa, since they’re associated with the Simpsons house, and the game doesn’t let you store it in inventory.
There’s several threads about this on the ea forums and other sites too. For example: here
So what does this mean? Be careful about sending Lisa on her Trick or Treating task. I’m holding off until this glitch has been fixed.
And what can you do? Let EA know. Contact them through their contact-us page here.