As jrhone said, there are a few things that could've gone wrong. Most likely is that your e-ticket has become disassociated from your reservation. It's a relatively easy fix, but you are likely to have to visit the counter so allow extra time. You might call AA res, and see if they can see the problem. They should be able to reassociate the e-ticket to your reservation pretty easily. They could also identify what other problem there might be, if any. i.e. your ticket accidentally got issued on paper.