Given so few details the generic advice is to make it smaller. We don't need mice, keyboards, more than one stick RAM, in some cases no video care to use onboard video, and keep going as no case is required.
At some point it will power up.
However before all that, unplug ALL USB things and the main power connection. Press and hold the power button for 60 seconds, release and only add the main power connection. Test. What happens?
In many of these machines the owner didn't realize the CPU socket is fragile or you must never touch the CPU pads or socket. If they get too bent up you install a new motherboard.
Just as the title says. I tried installing a new cpu today, but it is not compatible with my motherboard. I install my old one again, and now my computer does not have any display, no power to keyboard, but my mouse turns on. I switched around the USB ports my mouse and computer use and my mouse still gets power from the port, so I don’t think enough power is getting through. Could this be a cpu issue? Or could I have accidentally knocked something?

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