It appears the nameservers for Stack Exchange are returning SERVFAIL for all queries, meaning most users are unable to access any Stack Exchange site, or will shortly lose access once their local DNS caches expire.
Things should be coming up again shortly. We recently relocated our NY datacenter, and therefore all the IPs in NY changed.
We didn't update to the new IP for the master DNS server (in both the named.conf and the zone files), and therefore all of our slaves hit our expiry time (one week). This is fixed now, so things should start coming back up.
We apologize for the oversight on our part.