When I plug in my pc it turns on, not when I hit the pwr button

[Anthony_Mazio1-]

When I plug in my freshly built computer it will turn on for a quick second, then turn off then turn back on,  when I hit the power button nothing happens, it only turns on when I plug in the pwr supply, I get no display on the monitor though, this build is using a hp 7420 motherboard.

[itsmyname]

@Anthony_Mazio1-  -- When I plug in my freshly built computer it will turn on for a quick second, then turn off

For some brands of computer, this is normal, e.g., my Lenovo desktop.  This happens when the computer is connected to electrical power. It does a quick POST ("Power On Self Test"). If the POST successfully completes, the computer powers-off. If the POST "fails", then the computer starts "beeping".

then turn back on, 

Most motherboards have an option to "after the end of a power-outage, automatically power-on".

This is good when you want your computer to be a "server" that will reboot after an power-outage.

Is your computer configured in this way?

when I hit the power button nothing happens, it only turns on when I plug in the power supply

OK.

I get no display on the monitor though

Something is wrong -- the POST might be "failing".  Do you get any "beeps", or flashing-lights on the lamps on the computer's case, or on the lamps on the keyboard, in some sort of pattern?