Also, I seem to recall that these displays didnt have a hugely long lifespan/ various issues. It may well just be dead...
I'm just puzzled that the power on still works, though its possibly on a different circuit to the monitor.
You're correct. The 17" Studio Display was known to die if you simply gave it a stern look.
The power-on is part of the USB circuit. The front-panel touchswitch voltage is derived from the 25vdc standby that also is the main power for the monitor. It's not unusual for it to be working without a visible display.
The most likely scenario is the inverter board in the monitor that creates AC from the 25vdc to power the two CFL tubes that light the display has crapped out.
Try this: Power up the computer and shine a small flashlight (NOT your phone - that won't work) at the display at an angle. Look closely and you'll probably see the display is working and simply has no illumination. That being the case:
I would say Google "Apple M7649 display inverter" and you'll find more than you ever wanted to know about that board including that it's pretty-much non-repairable, places to buy a replacement ($50–$70-ish) and complete instructions to do the job. EXCEPT:
Since you bought it that way, you don't know if the inverter quit OR if both CFL's burned out - that would be when one dies and the owner keeps using it with the other one cranked all the way up until it quits too. SO:
It will be a safer bet to buy a working display (lots of them on fleabay $60–$150-ish) and keep yours for parts since it probably either has good tubes OR a good inverter AND has a good leg. That's because the way the leg sticks out, they get broken all the time when idiots toss them in a box with a couple sheets of rolled up newspaper and expect them to survive shipping. Unless it's in the original box (almost never) that leg
should be removed before shipping (M6 Torx bit) OR VERY well packed to protect the leg.
Or you can just use a VGA monitor……