Make sure the 4 or 8 pin power connector located near the CPU is plugged into the motherboard (MB) and is fully seated.
Make sure the 20+4 pin main power connector is fully seated into the MB, and is fully seated.
Make sure the PCI-E 6 or 8 pin power connector is plugged into the GPU video card; if the card has such a connector(s), and that it/they are fully seated.
Make sure the Video card and RAM is fully seated in it's slots by wiggling it to ensure a good connection.
Make sure you used the standoffs that came with the case to mount the MB and not directly to the case. [note: Some inexpensive cases used raised nipples in the sheet metal for direct mounts instead.] ...... in either case, make sure the standoff's / Nipples (love that word) are not going to contact and Solder joints on the MB as sometimes MB's are soldered wrong and they short out.
Make sure you used the rear I/O port panel that came with the MB, and not the one that came with the case. Make sure none of the little tabs have shorted out the ports.
If you cannot get into the BIOS on an older PC, try a PS/2 keyboard, on a newer PC .... "Please plug both Keyboard and mouse into a USB 2.0 ports (black) and not the USB3.0 as usb3 only works after drivers are loaded in windows" (H/T FacistNation).
Try one stick of RAM, and then another, make sure the RAM is fully seated by wiggling it in it's slot, Also be sure it is Set to run at it's proper Seed (MHz), Latencies, and Voltages in the BIOS.
Make sure there are no empty power plugs shorting out to the case.
Make sure the CPU fan is plugged into the CPU_FAN header.
Try an External build, tear it down to a minimal functioning system, lay the main board on a piece of plain brown cardboard , with only Keyboard, one stick of RAM, CPU and PSU to rule out shorts as well as bad components. [Temporarily use onboard graphics if available otherwise use your video card.] As to rule out any bad components replace them one at a time inserting them so you can see what part is possibly bad.
Please be sure you listed ALL of you hardware components SPECIFICALLY so we can better contribute to your dilemma
Make sure the PC is plugged into the wall (gotta ask) Take the 24 pin connector loose from the MB and jumper the green wire with any of the black wires, as long as the jumper is inserted the PSU should stay running.
If the PSU ran during that then take the front panel connector loose from the MB and jumper the power switch pins for a few seconds, if the PC starts the front switch is bad or connected incorrectly. If it does not start doing that then either the MB is shorted to the case, or it is bad.
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