iPad or iMac can connect wirelessly to router (airport extreme) but not both

I have airport extreme router. I have iMac with airport. iMac was working wireless just fine or with Ethernet connection. I bought I pad, brought it home and tried it. It connected to the network after putting in the password. Now the iMac can't connect wirelessly, either at the same time or when I have iPad turned off. aAnyone know what I have to do to solve this problem?