i was Checking My Facebook Homepage Suddenly My Screen Turned To Black , But The System Was On and the it was connected to power, And when I pressed the Keys down ,the screen goes back , but there was no brightness back of it ,after 0.1 second the lcd turned completely black , and there was no screen on it, and no keys could work I pressed the power button for a few second , and my macbook turned off , then i disconnected my external hard drive ,so i turned it on again , and system started with no problem and the light of lcd was normal , but after 10 second it turned black again , this time there was no screen on lcd , i disconnected All cables attached to my mac , and pressed down the power button ,the screens loaded (like when you connect your mac to power )and it went back with normal light .and now i am working with my mac for 4 hours , but nothing abnormal happend , and it is working fine , should i be worried about this problem ? how can i know my system health is fine ? could it happened by The Electricity or power problems ? could it be a software problem ? or it is the lcd light problem ? please help .. < sorry for my mistakes in this post , my english is not good at all ) 