Try rebooting your entire network. If it's a modem+router combination device, power it down along with the Roku(doesn't matter which one you turn off first). Then after 2 minutes, power on the modem+router first, wait for all the lights to pop up then turn on the roku.
If it still can't connect to the internet, you may want to change the DNS on your router. The link roku5 provided should provide you a lot of information.