Hello !!
It might be corrupted DNS cache. To clear the DNS cache in Windows7, Click Start > All Programs > Accessories > Command Prompt. Right-click on it and select 'Run As Administrator'. Type the following and hit enter:
ipconfig /flushdns
Download ATF Cleaner
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Click "Main" > check 'select all' (except for Prefetch) this first time using it, then click "Empty Selected". Do the same for FireFox or Opera if you use either of those browsers.
Finally go to Control Panel > Internet Options.
On the General tab under "Temporary Internet Files" Click "Delete Files".
Put a check by "Delete Offline Content" and click OK.
If that all fails then possibly...