No Built-In Display, but external video works

It sounds like you have an inverter problem.

I’ve had this popup with several older Mac Powerbooks. It’s cheap to replace the inverter but a pain to fix.

The inverter is inside the display itself, so you need to take much of your Mac apart to get to it. Make sure you get the correct inverter for your model. Apple tends to change parts with each new iteration of its computers, so the inverter is specific to the Mac.

Once the inverter is installed, however, your screen should work well again.

OK, sorry to bother you.

Lets see if we can get some diagnostics: Using a flashlight shine it through the lids Apple logo are you able to see the faint outline of your desktop icons? If you do then the fuse link and/or buck converter.