Sometimes routers lock up. Try shutting down the router (unplug if there is no on/off), wait 30sec, then turn back on. See if that doesn't help.
Next try changing your DNS settings to something other then the ISP's settings. To change you need to get to the network properties, and change the TCP/IP settings. Best way is to click on START, SETTINGS, CONTROL PANEL. NETWORK CONNECTIONS. Right click on LOCAL AREA NETWORK (if you are physically connected), and go to properties.
Once there, scroll down to INTERNET PROTOCAL (TCP/IP), highlight it, and click on PROPERTIES.
Here is where you can enter new DNS settings. I use 22.214.171.124, and 126.96.36.199 respectively. You can do a search for DNS servers on the internet and put one in there to see if it solves your issue.
Just a NOTE: Recently ISP's have been updating their systems and DNS servers due to a flaw in the DNS design which is world wide. If your issue of reaching some sites fails and is recent, it could be due to this update process.