iMac 27" 5K Screen Not Working

Hi there.

We have a desktop Mac and one day the display won't turn on.

When we turn it on, it makes a small click sound and then it sounds like it's running and the fan is running but nothing happens with the display.

The keyboard will not connect to it and it won't charge to it either.

Plugged the keyboard in elsewhere and it worked.

Tried to use another keyboard and plug into the USB and that didn't work either.

Also have turned it off, unplugged for hours, plugged back in and tried turning on again.

Same thing - sounds like it's running but nothing on the display.

Any advice or tips would be greatly appreciated!