Yeah, if you've just physically broken them, you'll need a new chassis.... if you've shorted the connections on your MB, you'll need a new MB (though you'd know that by the PC not working properly anymore anyway)
Hey Dolby - if the front ones on the chassis are broken, you could try and find a PCI backplate the has 2 USB ports (this one from Sybaritic), dis-assemble it, then screw them onto the front of your chassis, replacing the broken ones...
Go to any PC shop and ask them if they havnt got any chassis usb "spares" lying around. Usely its just a small PC board that connects to your output usb connections on your motherboard which in turns screw onto the front of your chassis. I had a spare one a while back and flogged it for R75