Well, first, the motherboard LED really does not mean much. The small, always on, standby power supply inside the PSU powers the LED. It has nothing to do with the main circuits that provide operating power to the PC.
About all it does is tell you that the system is plugged into a live wall socket and the PSU is physically switched on.
Second, our standard checklist and troubleshooting thread:http://www.tomshardware.com/forum/261145-31-read-posting-boot-problems
although oriented toward the new build, still has some useful troubleshooting ideas.