The good news is that you probably won't have to rip out everything. You certainly will want to either rip or tink back past the mistake, but you should be able to leave all the stitches below on the hook.
You might check out the fixing mistakes videos (http://www.knittinghelp.com/videos/knitting-tips -- scroll down to find it) to see if any of these look like your mistake.
Most of all, don't despair! You can fix this without having to start over!