My Dimension 5150 box will not turn on! Just

- disconnect the power cable of the box from the back of the tower and press and hold down the power button for 10 seconds (remaining power will be depleted).
- Now try using a different wall outlet.
- Check the diagnostic lights to see if the specific problem is identified (the computer has four diagnostic lights labeled "1," "2," "3," and "4" on the front panel). Also, see "Power Problems" in your computer Owner's Manual and/or Service Manual.

http://support.dell.com/support/edocs/systems/dim5150/en/sm/tshoot1.htm#wp1120147

The orange light is a diagnostic indicator that something is wrong. Most likely it is a bad power supply.
Unplug the power cord from the back of the PC and wait 5 seconds.
Plug the power cord back into the PC and listen for power supply fan to kick in. as shown in this picture.

If the fan doesn't come on then you have a bad power supply.
If it does come on then you have problem with the motherboard.
Either way, you'll need to have it repaired. Hopefully you are still under warranty.