From your description, it sounds like the modem has a router built into it. If that is the case, it is possible the old wireless router was not used/needed, or it could be that the wireless router was acting as an access point only (basically bridging the wireless devices to the router without doing any routing itself). To give you some guidance, I would need some additional information.
1. You stated that the computer is currently connected directly to the modem via Ethernet cable. Has that always been the case, or did you rewire after the wireless stopped working?
Forgot 1a. How many ethernet and USB ports are there on the back of the modem?
2. What is the ip address of your wired computer?
3. What make and model cable modem do you have?
Edited by InterestinglyAverage, 02 February 2009 - 11:54 AM.