When you turn a system on, and it beeps, sometimes also blinking the power/sleep light, that means there is a memory issue, so the system can't start up.
Try swapping your memory cards, and try each card individually in each slot to see if there's a combination that will work. If not, find some known working memory and try that. If that doesn't work, take it in for service.
Here's how to access your memory: http://support.apple.com/kb/HT1651