Best way to handle it is to backtrack your own actions.
Did you touch anything other than the videocard you mention.
-If so, what were they and are you sure you didn't by accident disconnect a powercable that's supposed to be there ?
Did you use the videocard on your old motherboard (assumption false)
What you said yourself, either one of your other parts is showing malfunctions, or you simply missed something while operating the Motherboard.
I can tell you from my own experience though, having a case where the mother board is simply not placed high enough (2 milimeters for me ) and you force it, it could cost you the life of your equipment.
When you do reopen it, make sure you have a table, clean, with plenty of light and room to put your parts, so you can backtrack every step you take.