A few people reported overnight that they went into their Springfield and found that all their hearts had disappeared. A similar thing happened to me with the coins during the Christmas event.

Over at the official forums, someone has suggested a solution. You can see the thread at http://forum.ea.com/eaforum/posts/list/9282854.page , but the suggested fix is to leave the game, close it in your multitask bar, and then go back in.

Let me know how it goes.