I would double check your cable connections first. This could be a connection problem, bad cable, the back light could be out on the LCD or the problem could reside on the motherboard.
Please give us some more history of this machine, including but not limited to why you replaced the LCD.
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From the history you have given the problem either is in the cable or the LCD backlight. I would contact who you purchased the screen assembly from and inform them of the problem prior to attempting to change out the LVDS cable. They should take it back or give you a partial refund. Hopefully you still have the old screen assembly with the known good lvds cable attached. '''Please note:''' Most of the cables that go through hinges on laptops break internally. Thus they can look new and still be bad. The LCD in the old screen assembly also has a good backlight. Replacing that will require opening the LCD a feat many people do not feel comfortable with nor confident about their ability to complete the task. I have done a number of them. The fore mentioned is why I suggested you contact the place you purchased it from first. Changing out cables may void any warranty you have. I don't want to give anyone advice that has great potential to put them in a worse position than they were prior to my advice.
I would double check your cable connections first. This could be a connection problem, bad cable, the back light could be out on the LCD or the problem could reside on the motherboard.
Please give us some more history of this machine, including but not limited to why you replaced the LCD.