Instructions
Make your Sims perform any other action to change their facial expressions. Some generic actions, such as watching TV, will not require your Sims to change their expression. Triggering an exciting event, such as working out or making "woo-hoo," can have a drastic effect on your characters' facial animations.
Restart the game. This should reset the characters' facial animation back to normal.
Download an update or patch for the game. If there is a facial animation glitch that is stuck on your Sims' faces, then there is a chance that EA, the game's publisher, has already taken steps to fix it.