Why screen goes blank halfway through bootup

After screen started flickering during graphics program use the screen went gray then Mac Pro shut down.

Have been unable to start up since.

On pressing power button there is a Normal chime, Fans start, Apple logo screen displays but when it reaches halfway stage the screen goes black and then no further output from graphics card.

Have tried safe mode, reset NVRAM, recovery mode but cannot get past halfway through apple boot screen.

I think that the computer is running despite no video output.

Is this a failing graphics card ?

How can I check ?

Mac Pro 20019 5,1 with 2 x 6 core 3.33mHz Intel Xeon processors.

AMD Radeon HD 7950 graphics card with 3Gb VRAM