MBP startup issues

Hi
I've got some issues with my MBP mid 2012 15" retina (A1398) while I try to boot it.

I get the start up sound, but rarely anything else. Occasionally there is light in the screen, but mostly not. Not even the backlights.
The keyboard responds to commands, such as resetting the PRAM and SMC, but I don't think it loads as much for me to be able to log in. So I can't test if the backlight works as the keyboard is completely dark.

However the fan is running, which is my only indication that the Mac is on, other than the reboot sound.
If I turn it off by pressing the on/off button for more than 10 seconds I can start the process all over again.

I've tried to hug it up to a monitor, but that doesn't sort the problem.

Any ideas for me to bring it back to life?