You are connected to the access point, but the internet cannot be found?

I've been having problems with my wireless router lately. I have Cisco Linksys WRT54G wireless router hooked up to the main computer and I have a desktop computer in another room that has a USB Network Adapter from Linksys hooked up to it and it uses the main router hooked up to the main computer. Recently it stopped working for some reason and when I try to connect on Linksys, it tells me that "You are connected to the access point, but the internet cannot be found." I don't know what this means, but I've tried various ways to fix it and even called tech support and all that and nothing solved the problem. Now on the desktop computer that connects through the main router says it cannot connect to the access point. I don't know if there may be a problem with the main router or what, but we've tried a different router from a family member and it wouldn't work. We also have a laptop that connects through the main wireless router that also has the same problem and won't connect to the internet. Anyone have any ideas or suggestions?