Try power cycling your modem and router. unplug the power cords and set for 2min, then plug in the modem then the router. If your modem is setup for dynamic dns then it will replace the old DNS settings with new ones if they have changed. If that does not help then go into the modem setup page and manually change your DNS IP addresses.
Here are some good ones:
These are DNS addresses for OpenDNS and their servers have been setup to be immune to the recent DNS poison attacks that are starting to run amok on the Web.