You need to determine if the power supply is coming on. The keyboard flash is evidence that it tried but maybe powered down. You should hear fans running. If not, the power supply started and powered off. It will do this in response to an overload or underload. A fan that "twitches" and stops is an indicator that this is happening. If you can determine the PS powers on and then off, you will need to see if something in the system is overloading it or whether the PS itself has become defective. This is best done by disconnecting unnecessary devices such as drives, add in cards except the display card, etc. and trying to boot the machine to at least bring up the POST screens. You need to rule things out one at a time this way. Repost any findings. Good luck.