My laptop wont Turn On though 'Power On' indicator is lit

I was having problem with my Laptop's battery( backup was for maximum of 20 mins), today there was a power failure at home and the battery ran out. My system usually goes to ''Standby'' mode when battery backup is about to finish, sometimes after ''Resuming'' windows it again goes to ''Standby'' and again ''Resumes'' then it works normally. But today its not turning on, when I press the power button the ''Power On'' light lits up then the ''Lock'' light just blinks and the Hard disc light lits up. Then after 2 seconds the Hard disc light blinks and goes dark, the ''Power On'' indicator continues to glow. I waited for some 5 minutes for the system to resume but of no use, then tried the ''Hard Reset'' method which I read in some post, tried 6 times but still in the same condition. When I switch off the mains, after 45 seconds the ''Power On '' light also turns off. The screen remains black the entire time and doesn't show any boot messages. I tried to get into bios by pressing "del" key but of no use. I removed the Hard disc and then I am able to get into bios. What could be the problem?