Who can read Google's mind? It could be any number of things, but let me take a stab at a couple possibilities.
The problem most likely occurred with something you added or changed.
1. Did you change the phone number/business name/address? Each of these must be consistent with your original listing as well as how you are listed in the phone book.
2. Did you inadvertently create a second listing? Google has experienced "Google Local Listing Spam" where people submit a bunch of listings for the same business. Even one "duplicate" listing can mess things up.
3. I would not consider all to be lost. You could reappear just as fast. In the meantime, concentrate on getting as many reviews as you can as this is one aspect Google gives weight to. Also take the time to get listings in the other 25+ local directories. Again, use the same information in every listing to remain consistent.