I had this exact issue on an old gaming PC, PSU wouldn’t power the MB, but would be fine by itself. My GPU had a short, gods know how it happened. Juts keep disconnecting parts and try powering up again until you get the LED indicator. If everything is gone and it still won’t power on, it’s the MB. You could get a multimeter and measuring the power lines to see if there is a short (5V to ground, 12V to ground and I think there are 24V lines?). Maybe look for burst or burned capacitors and if you’re crafty and able to solder replace them. Otherwise replace the MB or defect parts.
I had this exact issue on an old gaming PC, PSU wouldn’t power the MB, but would be fine by itself. My GPU had a short, gods know how it happened. Juts keep disconnecting parts and try powering up again until you get the LED indicator. If everything is gone and it still won’t power on, it’s the MB. You could get a multimeter and measuring the power lines to see if there is a short (5V to ground, 12V to ground and I think there are 24V lines?). Maybe look for burst or burned capacitors and if you’re crafty and able to solder replace them. Otherwise replace the MB or defect parts.