I had this same problem when I was trying to change my battery just a few days ago.
The cause was as @christophe34554 described: I had broken off one of those small components on the motherboard near the display connector when I pried up the connector. This was the leftmost of 5 identical parts, each of which has 4 terminals.
For repair, I tried to figure out what they were. They were tiny little things (about 1mm by 1mm square). My trusty DMM showed a resistance of around 3Ω through them on the other 4 intact components. I replaced the broken part with a two separate 0201-sized 3Ω resistors, and that got the screen working again.