That if you have an Internet domain, you have to make sure that you renew the registration before it expires. There are companies out there that keep an eye on the domain registrations, the second it expires they nab it, then hold it for ransom. They then sell to the highest bidder, so if someone else is willing to pay more for it, you may need to take legal action to get anything out of it, and may lose the domain anyway. In the OP's case he was unable to re-register before it expired and lost out.
These days when your domain is often part of your business identity, and you can spend thousands of dollars promoting it, and identifying with it, the fact that someone can legally steal it from you and sell it at a profit means the domain registration system is in some ways, broken.