You have not given us enough information to help, a computer not coming on is not the same as a black screen. Do the fans start? or is it totally dead?, is the hard drive activity light flashing?
It could be bad RAM, bad motherboard, bad screen, bad PSU
You haven't the skills or knowledge to diagnose the problem, you will have to take it to a workshop to find the problem, try to find one with no fix no fee policy.