need help with my laptop

hi my dell laptop was giving problem with my cpu fan.
sometimes the fan worked fine but sometimes it didn't work so my laptop shut downed.
i opened my laptop casing and tested the fan by disconnecting the fan and connecting it again but it didn't worked. so i got i sense that the fan wasn't working, but when i closed the laptop and booted it up
charging light was up,power light was up but the hard disk light didn't lit up. after i pressed the power button the the lights lit up for almost 20-30 seconds and went off while the screen was always black..

what might have been the problem please help me out...