It powers on and then turns off after 30 seconds

I bought it to repair. It had a flashing orange light when plugged which isn't normal. I disassemble it I saw an immediately problem. The battery had the positive wire reversed with the negative wire. I think that's what caused the problem. I reversed the wires back. It now charges with a solid orange light and the battery is holding charge but the screen is still completely dark no backlight at all. Any suggestions would be appreciated, thanks!