Most cable modems authorize by the MAC address it is connected to. Here are the steps to troubleshoot:
1: Connect directly from the iMac to the cable modem with your ethernet cable.
2: Restart the cable modem
3: Check and see if you can connect to the internet
4: If yes, connect to one of the PC’s and restart the cable modem
5: If you can connect to the net then, the weather may have damaged your AirPort router. You can try resetting it to factory defaults by pressing in the reset button on the back and holding it until the status light flashes. You can also select “Reset to Factory Defaults” from the AirPort Utility.
Hope this helps.