booting trouble on laptop

Shut laptop down last night. When trying to start booting this AM all I got was a black screen. Noticed that a single yellow light came on, there is normally a green one when starting the operating system (XP). The light is one of 4 located at the base of the laptop. I then disconnected the AC adapter and battery. Then connected the AC and battery again to see if that would work. The single yellow light came on again. Went through the same reconnect process, after waiting 5 minutes, and then 2 green lights came on and the laptop started to boot.....After completing the booting process all 4 green lights came on and I connected to the internet....Can someone tell me what's wrong and also how to fix?

Thanks