Has this been stored a long time? If so, there are electrical contacts that affect startup that are likely oxidized:
- Remove the internal backup battery ("PRAM" battery) and set it aside† for now.
- Remove and reseat all RAM modules
- Remove and reseat graphics cards and other PCIe cards
- Disconnect the cable from the power supply to the logic board and reconnect
- Disconnect the power and data cables from hard drives and reconnect
Only then try starting is Safe Mode. Your Mac Pro is a Intel Mac, so use that instruction set. If the computer started in Safe Mode. let it sit in Safe Mode for about 30 minutes to insure all disk repair that SM can do is complete. Then try a normal restart.
If you have the original system install/restore disks that shipped with the computer, try booting from CD/DVD 1, holding c while booting.
† — if the computer was stored for more than a few weeks with the PRAM battery installed, the battery is surely extremely dead. Replace it using a common 3V CR 2032 lithium battery.