Hi, the car suddenly lost most of its power and was sounding like I blew a gear in the diff or pinion was stripped, but then I could smell electrical burning so immediately pulled the battery. At first thought , I thought it was the motor but the smell was coming from the ESC.

When I plug in the battery the led lights up solid green and stays that way (even with the power switch off ). Turning the switch on, nothing happens. No power to the motor. Servo is still working. So the radio is ok.

I can't seem to find what the solid green light means.

​​​​​​Any ideas would be much appreciated.

Try re-calibrating, does the light on the ESC indicate throttle changes?

Red LED on ESC after turning the ESC on means abnormal throttle signal. Given that you smelled something bad from the ESC, I'd say it's toast. Probably a power mosfet which would explain why you lost power. The BEC circuit is separate and that's why the servo still works.