well - if the screen lights but nothing else, then power somewhere aint working would be a guess.
otherwise you have two problems - one disk and one display ribbon.
i would go for power first - not sure what to suggest for you, but i have had problems with my 540c in the past where the power was simply not working, then it kinda was, then it worked properly.
do things change when you wiggle the power plug? pull it out then stick it back in again?
my 540c can have problems starting the disk from a cold boot, but i get a weird alarm siren sound, then it kicks in - anything like that happen. its not a chime or bell sound, its more like an emergency siren.