Sounds like an incompatibility. It is possible you mistakenly installed a PCI adapter into a PCI express slot, or vice versa? (to be honest, I don't even know if that's physically possible).
It’s always possible as well that the PCI slot has gone bad, or somehow corrupted. Did you try any other slots? Or maybe the contact pins on the PCI adapter need cleaning. Or maybe the PCI adapter is just bad, a dud. Lots of possibilities.