clear the internet cache
cycle the power on your router (the DNS resolution could have fallen over - it does happen from time to time - turning off/on refreshes it)
check the DNS settings on PC and router
which ISP are you with? Could be fault there - for instance Zen's DNS servers went partially titsup last night. I could only connect if the PCs were set to use OpenDNS
faulty router? Any sign of it overheating?