Answer to "No video on lcd, video ok on external monitor"OK I had this exact same problem (and I am a hardware tech, fyi). It occurred when the displayport jack was inserted crooked - the entire machine shutoff and then only external display worked. I tried everything - Apple Service Diagnostics didn't help, neither did clearing PRAM or SMC. Then I obtained schematics for the Macbook (this is a 2009 2.53Ghz Core 2 15") and replaced the MOSFETS and power switch that the Displayport uses. Still no dice. Finally, I booted it with the internal display unplugged. Then power cycled with the display plugged in and...it works! *scratches head* - these computers are a mystery to me at all times. It must have reset some crazy SMC MUX detect or something(?) I honestly have no idea. But worth a try.