Remove all PCIe cards and riser cards.
Try restarting the server.
If the server starts, reinstall the PCIe cards and riser cards, one at a time, to isolate the defective card. Restart the server after reinstalling each PCI riser card.
Remove all DIMMs and restart the server.
If the server restarts, reinstall the DIMMs, one pair at a time, to isolate the defective DIMM. Restart the server after installing each DIMM.