How do i get the pci pnp to work again?

I was trying to install a modem in a Shuttle Xps computer, but it did not
want to connect to the dial-up site. So I installed a second one that did not
work well in the computer. But installing the third one in the same slot
(there is only one pci slot in this computer) did not get recognized. And now
it does not recognize any pnp cards in this slot. What can I do? Is it
something in the services, or does it need something else to be done?