Did you disconnect the battery from the motherboard, before replacing the screen?
Are you sure that you got the correct model screen as a replacement, e.g. is it the same make and model number screen?
If they're different, find the make and model information on the back of both of the LCD panels and search online for (insert make and model) datasheet for each screen to hopefully find them. The datasheet will have the pinout wiring connections so that you can make sure that the new screen is the same as the original and that it is compatible with the motherboard's video connector wiring.
If they are the same or if you can't find the datasheets, what the make and model number of the motherboard(printed on motherboard) as the schematics will be required to find out if there is a fuse or not in the backlight power supply? Some motherboards don't have a backlight power fuse. They turn a power mosfet on and off to supply the backlight power.
Post some clear close up images of the motherboard in your question, especially showing the area a couple of inches either side of the video cable connector. If there is a fuse maybe it can be found. Here's how to iFixitでの質問に画像を追加する方法