You were working on the PC with the power cord still plugged in?
Did you try pulling out the network card and rebooting? If it works after pulling it out, you might try plugging it back in (and make sure it's plugged in snug). I've had times when I've plugged in a new part for the first time, and the computer wouldn't boot. But then I took the new part out and plugged it right back in, and the computer booted fine.
Btw, you're right in the assumption that you would simply have two network cards, though with one obviously not working. It's been common for awhile for a network port to be included right on the motherboard.