Just to make sure I understand you correctly.
The 1200 watt PSU is confirmed to be working on its own?
But when you connect it to your PC, the system won't boot, but acts like you described in your first post instead?
Despite the possibility of coincidence, it is difficult to ignore that you rewired the power button, right before all this began.
Have you checked, that all the front panel connectors are connected in the right places? I'm especially thinking of the PWR_SW and the Reset_SW, but also check PWR_LED and HDD_LED
What happens if you press the reset switch when the PC is off?
Make sure you didn't reverse the wires when you reapired the power switch.
If you soldered the wires to the power switch, are you sure they are not accidently causing a short of some kind?
Check all connections in your PC. Are all cables from the PSU connected correctly on both ends (PSU and Motherboard)
EDIT : What was the actual reason for the repair you did, and how did you perform the repair?