The power supply connect with the motherboard in more than one place. If “the wire from the power supply plugs into the motherboard” is a thing, that may be your problem.

Unplug anything that is optional (that includes disks and stuff that you have more than 1, like maybe memory sticks) and try to power the computer. If it works, replug stuff one by one.

As BigMikeInAustin said, remove everything and plug it again. I’d add to get a paint brush and clean stuff a bit in between if it’s dusty. Make sure to clean plugs and contacts, try not to clean electronic components.

Try to remove the board’s battery for a few minutes and turn the computer on again. Some times that’s enough.

Also, turn the computer on for a minute or two, turn it off, unplug everything, and touch the surfaces to see if they are hot. Try not to touch any printed circuit board.