On my local network, I have several workstations that suddenly (overnight) lose the ability to connect to the internet. Some of these were powered on, some were off. Some are running XP, some are running various flavors of Windows 7. I have a cable modem that is connects to a Cisco PIX 501 firewall/router, which then is connected to an access point and a switch. Initially, none of the systems were able to connect to the internet (but all could connect to each other). I reset the PIX and had my ISP clear arp caching for my connection. This got my domain controller up and working. The domain controller also hosts my DNS server. I was able to get my primary Win7 workstation to connect after going through several diagnostics (netsh int ip reset, netsh winsock reset), and changing its IP address. I had two laptops (WinXP and Win7) working, but after a reboot, they no longer connect either.
I checked the settings on my PIX with what they were six months ago, and it has not changed. There was a very odd blinking pattern on the PIX this morning, but it hasn't repeated since I rebooted it. When I ssh into the PIX, I can ping IP's on both interfaces. My clients that cannot connect to the internet can resolve names, so I know they are connecting to the domain controller and getting a response back.
At this point, I am thinking that there is something flaky happening with the PIX hardware, but I am open to suggestions.