I would tell you to store the Simpsons house - which is the usual solution to these stuck character problems - but you can't do that. The Simpson's house can't be stored. So you're gonna have to do something a little harder.
Tap on the Simpsons' house. A box will pop up showing everyone who's working there. Write them down.
Now you're going to have to track down the building or decoration associated with each character, then store that so the character is stored. If you don't know all this you'll have to look it up. One place to do it is here.
Once you have them all in storage you need to synchronize your game by going to your Neighborhood and then returning to your town. Either that or completely close your game and restart it.
If you were not able to store any of the characters then I'd try waiting until they finish the task and see if that fixed it. If not the only thing I can suggest is the following, which worked once for me:
Tap the Simpsons' house then quickly tap the edit button (yellow with four arrows), grab the Simpsons' house and drag it as far away from the characters as possible. Once you place it then quickly send al the characters on task. You may then find that they show as being on 2 tasks: the task they were sent on and the one at the Simpsons' house. Wait until they finish and see if that corrects the problem. Depending on how fast you can grab and move the house you may have to do this multiple times.
If none of this works you'll have to contact Customer Support and hope they can help you.
In future never send more than the required number of characters to a building. This problem usually only occurs when you send an excessive number of characters on the same task.
