how old is this pc?
what model is this unit?
what display ports are you using to connect to the monitor?
What GPU are you using? (If any)
Have you tried to reset the BIOS?
Do you you usually have a RAID configuration?
Does the computer make and sounds at any time when you try too boot?
Can you smell a burning smell from the back of the computer?
Do you have any external devices connected?
Is this running through a UPS?
Is there a disc in the optical drive?
Answer these questions and we may be able to help you,