Many of the troubleshooting tips do apply though whether new build or other.
And the fact that is was working and now isn't, is helpful information. More helpful may be specs and the last thing that was done successfully on the computer (before it decided not to start)
if it worked fine then stop working i would start by having the power supply tested or swapping in a know good one. if it wont post then i would bare bone the pc no cards and one stick of ram to see if the mb will give a cmos beep code.