No screen can be caused by many things, but below are some common causes.

If the BIOS is bad or missing, the system cannot load anything and so you get no video output at all.

Bad power rails or slow bring up from one, or lack of current, can all cause black screen.

If the Power-VR2 (IC15, IC110) are jammed, faulty or disconnected, the system will not boot, and it will give black screen.

The system continues to monitor for the fans to work again then resets the system once they are ok.

The custom Sega 315-6289 Processor (IC105) is required to be fully working, along with its RAM (IC106 to 112), in order to get at least black screen.

If any part of this process or RAM is shorted, faulty, or missing you will not get to black screen.

The core CPU Hitachi SH-4 (IC1) must be fully working, along with its RAM (IC9 to 12), in order to get at least black screen.

If any part of this process or RAM is shorted, faulty, or missing you will not get to black screen.