So what I did was, I opened it up and found seven 25Volt; 220 μf electrolytic capacitors that were bulged at the top. That's a sign of a bad capacitor. I replaced them and now it works.    
  So this seems to be a problem specific to Dell U2711. The time needed to keep it unplugged before it works again grows longer and longer. Last time I had to keep it unplugged for more than 12 hours. I was reading some articles online it may be some capacitors that are failing on the power board.
U2711, will not power on