Why is my micowave not heating food anymore?

I've only had this Sanyo microwave for three years. It was brand new when I bought it and it was working fine, up to today. There's nothing wrong with it otherwise. It's not like I dropped it or mismanaged it in any way. It actually looks the same , as when I purchased it, so I would hate to think that I now have to get rid of it, for another one. What can I do, or will it cost me more to get it fixed then to buy a new one?