Make sure your sim is not doing an activity and is not in the middle of sitting down or going up/down an elevator/stairs/ecalator. In the Sims Tracker, you should have a picture of a house with an arrow on it (where the whistle would usually be). Click on that and your Sim should walk back to your own neighborhood and into his or her house.
If your sim is stuck in between some furniture and cannot "walk" out of the neighbor's house on their own, while in your neighbors' house, click on the town map so that you are no longer in their house. Go to your Sim Tracker and follow the steps outlined above. This should all work. After a certain period of time has past in real time (approx 24 hour or so), your sims will return on their own to their own house once you visit them while they are in their neighbor's house.
