http://support.dell.com/support/edocs/systems/dim4500/index.htm is the top of the documentation so follow that to http://support.dell.com/support/edocs/systems/dim4500/solve.htm#1153217 where it writes "If the power light is amber and green ? The computer is receiving electrical power, but an internal power problem might exist.
* Ensure that the microprocessor power cable is securely connected to the system board.
* If the problem persists, contact Dell. See "Contacting Dell" in the Dell Owner's Manual for the correct number to call."
Not to hold your hand here but wander around the documentation and find that connection and with the AC removed from the machine unplug and plug it back in and retry.