Screen becomes very dim, almost black

I replaced this guy's phone screen, it looks all good. two days later, he said when charging the phone will not turn on. But you can feel the vibrating. I re-insert all plug. It works. Two days, it happened again. This time I unplug all wire (LCD, Digitize, batter), try to turn on, the screen is very dim. I can just see the apple logo, which means the phone is turning on. But after that, I can't see anything. I put another screen on. It's the same.

Don't know what happened. Need help.