Mac Pro screen stuck on black screen

but are you connecting to the same ports they connected to in the shop?

I have seen a MacPro graphics port go bad, but the rest of the card worked FINE. I added an adapter to a different port and I was back in business.

The simplest thing your Mac can do to show a default picture on the display is to start up in Startup Manager, whose code is all in ROM, and no drives need to be working to get the display to work.

Hold Option at startup, and it will, over the course of several minutes, draw a gray screen, then add an Icon for every potentially bootable drive it discovers.

provided you can get something, it can be changed to a setting that works for your display

Then the output port of the graphics card, your display, or its cabling has likely failed.

Does the display have "on-screen display" settings you could look at to see if IT thinks it can draw something?

Your first step should be to secure your data, issue likely isn’t with the Mac itself but with your local setup, maybe the display connection.

Double-check that all cables are securely connected and that the correct input source is selected on your monitor. Try using a different display cable or port, and if possible, test with a different monitor.

Bad cable, or bad monitor, or maybe a monitor or controller that has lost its display resolution settings.

Or bring it all into the shop, monitors and cables and all.