black screen help

Hi to all here i am new an i hope this question has not been asked yet but here we go.. i have an acer aspire one netbook running linux, and just recently i started getting a black screen when booting the computer up. Everything seems to be running fine except the screen and when i hook up to an external monitor it does display a picture! but this is where im stuck, ever since i installed linux when using my laptop with it plugged it it would go directly into sleep mode when i unpluged the power. i found that if i boot my computer up with the power plugged in an wait for it to automatically sign onto my desktop and then unplug power, let it sleep then wake it my screen all of a sudden works?? i cant find anything thing online about this an would be very thankful for any input at all. thanks

and i have tried holding the power down with battery out and power unpluged but still did not work. thanks again guys