if your dell desktop PC is flashing amber light when you power it on, but not starting up, then chances are your power supply is faulty, but in some cases even after changing the power supply doesn’t solve the problem, in this case following image can help you,

Dell Desktop blinking amber power Light

all you need to do is, take out the green cable from 24pin power switch, and scratch it from middle, remove the plastic & take wire from an old power supply unit, or any other electric cable, scratch it from both ends,
solder one end of this cable with green scratched part & other side of that cable attach to the CPU body, anywhere in the pic above i soldered it with PSU body,
and then plugin the power cable and PC should start up automatically, your power button will be useless now, make sure after you shutdown the PC, take out the main power cable from PSU, otherwise your capacitors may burn ,
what’s the cause of this problem, as per my knowledge this problem is caused by some bad capacitors, those are not leaked but weak ones, with the time those capacitors will blow out and then you can change them, and after that i hope you can remove the cable patched with body.
Good Luck, try at your own risk,