I had left my Dell Dimension E510 on and came back later and it wouldn't turn on. The front power light was flashing yellow and no matter how long I pushed it, it wouldn't turn on. I disconnected the power for a bit and re-connected it and still the same. Someone said in a post somewhere to try removing the RAM sticks because they thought it might be caught in a data loop somehow. This didn't help. Someone else said it was hung somehow and that the wake up on lan was enabled. Finally, I unplugged the computer again and removed the quarter-sized lithium ion battery on the motherboard, put it back in, plugged the power back in and it powered up fine.