Hope you have Intel® Xeon® Processor E5520 (8M Cache, 2.26 GHz, 5.86 GT/s Intel® QPI) and Intel® Workstation Board S5520SC .
To resolve the issue, please follow the steps one byone
1. remove all external cards/HDD and boot the board without connecting DImms. If you are getting some beep code then board has no issue.
2. Add one dimm and boot the machine wihtout pci-e card.
If this setup boots then we can isolate the issue.
Which HDD you have?
BTW, try to clear CMOS aswell.