While it is connected to the external monitor, try using the hot keys to switch between the displays ( usually FN and one of the F-keys pressed together). However, there could also be a problem with the screen cable leading from the motherboard to the screen. I have seen these cable develop a short or just come unplugged. Furthermore, check the fans and exhaust ports on the bottom of laptop for DUST buildup. Dust will cause laptops to overheat and develop hardware failures.