We have a troubleshooting sticky at the top of the forum that was created just to troubleshoot this type of problem. The first thing to do is perform EVERY step in the checklist. It won't do you any good to skim through it, but if you take the time to perform every step the chances are very good that you'll find the resolution to your problem.
What was the problem? It's good to post what was wrong to help people who have a similar problem and look at your thread for a resolution. I'll take a wild guess and say it was #2 on the checklist, though.