I was using my XPS 400 just fine one day, then the next morning I go to power it on and it won't budge. The power button/LED just blinks orange (vice green when its powered on), the ethernet light blinks, and a green LED on the motherboard remains lit even when no power is supplied to the computer at all.

You should check the Dell forums for your issue. It'll be easier for you to find a solution

Usually a blinking amber power light indicates there is a failure on one or more of your PSU rails. Typically stripping your system down to bare essentials, trying to power on, will tell you wether it's a device causing the problem or the power supply/motherboard causing the problem.

Happened to see someone bump a old thread on the same problem I was having, and after reading through it seen the common factors were the PSU, Motherboard, or CPU. As its a system I was planning on recycling some parts and upgrading others and just throwing it in another case, I purchased another PSU for about 40 bucks and will try that first. If that doesn't work I'll try the motherboard next, then lastly, the CPU. Never going to go with a prebuilt system again, especially a Dell!