Could be a # of things.. does it do it for all browsers? A few things you can try.
-open command prompt (as admin) and type:
-clear browser cookies/history/temp files/cache
usually in browsers 'tools' menu.. if you use chrome, after you do this, close down the browser, open windows explorer, and type in C:\Users\You\AppData\Local\Google\Chrome\User Data\Default\Cache (assuming C: is your primary drive where applications are installed, and assuming Windows Vista or higher; if XP,i think C:\Documents and Settings\Users\....). If there's anything remaining in that folder, delete it.
-complete power cycle done on router/modem
should be in your router/modem manual, or look up online for your device
These may help. Now for some advanced tips, which I wouldn't recommend unless you know what you're doing.
~~ADVANCED TIPS~ (OPTIONAL)
-make sure nothing in hosts file is conflicting.
start>run, type "notepad c:\windows\system32\drivers\etc\hosts" (assuming u dont use xp or before i think? dont remember their paths. By default, you should have some text but every line should start with a # symbol.. there shouldn't be any line without a # that has text on it, unless you specifically block certain hosts. If you don't know what this is, prolly best to leave this one alone.)
-Changed DNS settings to Google's public DNS (prolly never have to do this since you DO have access and are able to browse the internet.. but as a last last resort, can try.. be sure to research this topic, write down ur old dns info b4 u attempt this, and if no change, then change dns back to what it was.. again, LAST RESORT)
only thing i can think of right now. May be a good idea for everyone to clear their chrome cache manually, as doing it from browser doesn't clear it alot.. I had around 840mb of cache before I cleared it manually.