Why my screen is blank

First of all, please verify the device you're working on; the previous entry you had just said P9 lite, and didn't even give a manufacturer (hopefully Huawei is correct). Apparently there's a 2017 version of this phone out as well and the screens aren't compatible, so you'll have to verify which one you have to see if any given screen will be right for your phone; generally shopping by the model number such as VNS-L21 will give you the best results.

In most cases the lack of an image on the screen can usually be attributed to a failed LCD or backlight. If you hold a flashlight up to the screen, can you make out any image at all? If so, then the backlight on the screen has failed, otherwise it's probable the LCD itself has failed.

In either case, the fix is to replace the screen. Fortunately iFixit has just the guide you need to make that repair yourself.

Huawei P9 Lite Display Replacement - iFixit Repair Guide

Huawei P9 Lite Screen