The most important thing to remember when troubleshooting stuff like this, is to always check the easier stuff first. Doing this can give you several benefits...

1) It can reduce the complexity of your troubleshooting.
2) It can reduce the time it takes to troubleshoot the problem.
3) It can save you $$$ when solving the issue.

So in this scenario, CHECK THE CABLE FIRST. All the suggestions here are good and could potentially be the cause of the problem, but CHECK THE CABLE(s) FIRST!

You'll thank me later. :)