MacBook A1278 Screen issue

Your lid must have gotten a good hit. What has happened here is the driver logic on the back of the LCD panel has broken free from the glass so the area that is black is not getting the signaling to de-phase the crystals so the back light is not able to get though. Here's a write up and a model as reference: How LCDs Work.

Sadly you'll need to replace the display. You may want to consider the easier way here of just replacing the complete lid unit Vs the LCD panel alone. Here's the two IFIXIT guides that explain whats involved:

• MacBook Unibody Model A1278 Display Assembly Replacement
• MacBook Unibody Model A1278 LCD Panel Replacement

While the complete assembly is more costly there is less risk of damaging anything, unlike the panel replacement which while being cheaper needs more tools and skills as you can break the cover glass.