|"all I did was swap cases"|
The logical assumption is that you did something incorrectly during the swap. Pretty tough for us to determine what. There was absolutely no need to remove the CPU. Hopefully you re-applied the thermal paste correctly (vertical line method).
Try basic troubleshooting - remove all cards, disconnect all drives, unplug all externals except for monitor & keyboard. Connect monitor to onboard video port (if available). Make sure both the 24-pin ATX plug & 4/8-pin ATX12V plug are connected to the board. Remove all RAM except for one stick. Double check that your front panel wiring is connected correctly to the board.