XPS8700 won't boot - displays "Out of range"

XPS8700

Intel Core i7 4770 3.40 GHz

Windows 10 64-bit

AMD Radeon R9 270 2GB GDDR5

In the middle of a flight simulator session the screen suddenly went grey with a loud buzzing sound. I shut down by holding the power button.

Since then, when I power on, there is no Dell logo and no boot sequence, and the Viewsonic display shows "Out of range".

The USB and the Ethernet ports are powered up,  but I cannot access the Setup screen by repeatedly pressing F2 after powering on.

Something has failed, but what? How can I troubleshoot and hopefully repair my PC?

Thanks for any suggestions.