Make sure that the IP address assigned by your router has not changed. Look for the WiFi address that will scroll up on the B2's display. That should be the same as the one in the addres bar of your browser.
I had the same thing happen to me the other day. I tried the Retry WiFi command, but it didn't work. Then I cycled power on the B2 and when it came up the second time everything was working again.