Portal trouble then. Replace the house. Evict the family if you want to keep them. Make a note of the house number; you'll only have numbers 1-10.
Quit the game, go to C:\Program Files\Maxis\The Sims\TemplateUserData\Houses on your hard drive and copy the appropriate Housexx.iff file to C:\Program Files\Maxis\The Sims\UserData\Houses overwriting the previous file. So for example, 8 Sim Lane is House08.iff and so on.
Now fire up the game again and go back into the house that previously faulty. It should now be working again; take note that if you put a sim with a job into the house the carpool won't show up on the first day in your new house (won't affect your job) while the schoolbus will.
Disappearing portals are a common problem with early versions of the game; later expansion packs included extra code to rescue the situation. Please see my Article on this site:
http://www.thesimszone.co.uk/extras/index.php?ID=74 in particular the "The Portals" section.