mysterious wireless internet connection failure

We have a Dlink wireless router. A pc is hooked up by an ethernet cable. A laptop connects via wireless.

Three macs all lost wireless connection to the internet at the same time for one week. Then all went on for two days and then off again.

I cannot find anyone who can give me a clue as to why and how to fix the problem!

The macs see the wireless network, but are unable to connect (usually they connect the easiest).

The message says something like dns failure. I can ping the router successfully, but cannot ping yahoo.com or other sites.

Anyone have any insight?

Thanks

Larry