iMac powers on but no video

Are you using external display? If yes, then try this -
   

• Put the iMac to sleep mode by pressing the power button on your computer or choose Apple menu > Sleep.
• Wait a few moments, then wake you iMac by pressing a key on your keyboard or clicking your mouse or trackpad.

It's an all in one, it has no video outputs.

Which iMac your display might be ...... Done for.check the brightness I.e someone playing a cruel joke dimmed the monitor,
Depending on the age, the backlight might have gone bad......
Depending on your version, I think some had an external connection to another display, connecting it, could help confirm whether the issue is with your built-in display or the video card, which is less likely.

Try resetting the SMC first, then the PRAM to see if the reset fixes it first.

Reset SMC on iMac:
Unplug power.
Press and hold Power button for a few seconds.
Release button.
Plug in power, then start up.

Reset PRAM:
Start the iMac
Press and hold command option p r

Boot with "T" key down, this is Target Mode and the simplest startup. An icon might appear on screen, this may indicate the video subsystem is marginally okay.
Boot with "D" key down this will run hardware diagnostics, tiny earth icon might appear.
Boot with SHIFT key down this starts with only basic Apple system.

The reset SMC is the first thing to try, unplug, hold power button 7 seconds, let sit for 20 seconds, plug power cord in, restart.

hi team, ok I replaced the board and now I have video but it shuts off after a minute or two of booting up. I can hear the hard drive grinding away and attached is the only thing I see.
imacerror.JPG

thanks