Here are the steps I would take. Powering the device on each time it seems logical and see if anything has changed.
-Check the power switch on the power supply. -Make sure the power cable is seated into the power supply. -Listen for any unusual noises when power supply is on -Open the computer and look for any codes or lights on the mobo, Google it -Check RAM is properly seated -Check all of the power cables visible to me from opening the case. 8-pin at the top, 24-pin on the right side, any hard drives/ssds, cable for graphics card -Inspect computer for any hot spots -Unplug video cable from graphics card the into the one on the mobo, this only works with some newish AMD, but most Intel processors are going to have onboard graphics -If nothing comes up and I can’t see any problems, you will have to start taking things out of the computer -Start with removing the graphics card -Check for any sort of damage to the power port and PCIe on the card and the slot Removing the card should allow better visibility of the mobo -Remove the back panel (if you need to) to see if all of the power supply cables are properly seated -Check any cable management in the back or any hard drives/ssd mounted back there
After this, if nothing has changed and I don’t have any more info I would probably take apart completely, check for any kind of damage, put back together and test again with the bare minimum and adding stuff as I go. If you don’t know how to take apart or put a computer together I would follow a guide on YT.
After that you should def have an idea what isn’t working, if everything looks fine it can be the power supply If the power supply seems okay and mobo isn’t showing any kind of lights or code (or signs of life) that is your best candidate
As you add stuff it should give you the best indication and if you are unsure, post another edit
My best guess based on your post is a dead motherboard but going through the steps like you want will avoid having to purchase an expensive thing for no reason