I suddenly can’t access my own web server within my LAN via the external WAN IP address

If I type the internal IP of my server into any browser the site works. If I type the external IP I get the DNS error.

So your web server is inside your network, but you cannot access it via the external IP address and other services can get through to it outside of your network?

My money is on some kind of NAT loopback setting on your router being reset or deactivated. NAT loopback allows you to access port-forwarded services—like web services on port 80—on your router’s external IP address as well as the internal IP address.

Is your router perhaps an ISP-supplied router/modem combo? It could be that your ISP reset or upgraded the router/modem firmware and the NAT loopback settings were munged in the process. Many ISPs reserve the right to remotely access the router/modem the provided you with to perform routing maintenance at times and something like NAT loopback is a fairly non-standard setting that could be easily overlooked—or ignored—during such a reset. Now I am not condoning your ISP reseting your equipment without your knowledge, but just putting that out there as something that might happen if you are using ISP supplied equipment.

I would recommend going into your router’s config to see if NAT loopback options—or something similar sounding—exists and see if adjusting that clears things up.

EDIT/UPDATE: Now knowing your router is an Actiontec MI424WR MI424WR (GigE for Verizon FiOS), I can honestly say this: The chances of Verizon mucking around with your modem/router without you knowing are quite high. When I had a residential Verizon DSL account a few “outages” ended up being “upgrades” I was not told about. And pretty much every contract/corporate gig I have had involved at least one incident of Verizon “updating” equipment remotely without telling anyone and then someone coming in the next day to clean up the mess; usually a simple modem reboot but it’s not “simple” when you don’t know what is happening.

All that said, I did find this nice page explaining how to configure a Verizon FiOS router/modem for NAT loopback which boils down to this; I am editing this to be a bit clearer and generic:

• Poking around my router’s interface, I searched for DNS settings. I found what I needed under the “Advanced” section.

• After confirming that I wanted to proceed, I clicked the DNS Server section.

• And added an entry for [my domain name] pointing to my router’s private IP address.

• I saved my changes, ensured that my computer was receiving its DNS settings from the router, and voila! I could now browse to [my domain name] from within my network successfully!