These situations can be tricky.
First, look up the domain record of your domain to determine the following things. You can look up your domain at InterNIC.
At this point you have two options:
Contact your sponsoring registrar if one is listed. You will probably have to go through some kind of process to prove you own the domain. Be sure to read whatever FAQ they have on their site to understand their procedures, and when you contact them be sure to mention that you’ve repeatedly tried to contact your domain registrar, as they usually don’t like to interfere unless they have to.
Transfer your domain to another registrar. This will only work if you have access to the email listed on the domain record, and the status of the domain is not locked or pending deletion. Be sure to use this option quickly because it can get problematic if you do it too near the domain expiration date.
Whatever you do, do NOT let the domain expire if you can help it. If you do, you may lose it forever, as many companies snatch up domains as soon as they expire in order to try to sell them at a profit.