|If nothing at all shows on the screen when using the radeon you might want to open the case and make sure any external power connectors to the card are hooked up--if the card requires them. Maybe the shop didn't reconnect everything.|
If that's the problem try booting up after making the connections and see how your system works. If it's still freezing it may be a power supply problem. Previously, did the computer ever NOT freeze when using the radeon? You could also check to see if updated drivers are available from ATI.
If you open the case and the connections are OK then maybe the shop was right and the card is bad, although it's kind of odd it died completely while they had it.