You may need to replace the cmos battery, its a cr2032 3v button cell
Make sure all your expansion cards and memory are reseated.
Make sure all cables are reseated.
your power supply could be bad
If that doesn't work try this
Here is my fix for the dell dimension 4400 amber light problem. It works!
replace cmos battery.
then unplug power cord form wall about 5-10 secs.
then plug it back in, wait 5 secs.
If amber light comes on, press and hold power button until it goes out.
light out, press and hold power button while amber light comes on and
goes off again holding power button 2-3 secs after light goes out.)
repeat whats in parenthesis until computer turns on
it should only take about 6, 7 tries.
After the computer boots, you may want to setup cmos or boot into windows.
After you shutdown, unplug power cord for 5-10 secs then plug it back in.
Press and hold the power button, computer should come on.
I found that's the only way to turn on the computer.
If you shutdown and don't unplug power cord the computer may not boot, mine didn't.
Strange I'd say