Maybe there are some problem in your start button (the button wire connect to mainboard is Loosening.
Please press the start button for about 3-4 seconds then see it would start. Always it may solve the problem, this happend to my inspiron 6400
too, after used for years the start button failure. It shows only power lamp lights green but the other light cant light red.
If no, maybe your must change a power supply, as the lower volt current cant start all the device.