If none of the sites, check the settings of the Internet connection. Change the settings by opening the menu "Internet options". Error when opening pages may arise due to your equipment - check the modem.
Page of the website may be unavailable due to low Internet speed - try to install another browser. Performance different, and this is one of the most important options for working in the global network. In addition, some browsers can block unwanted content sites.
Check the software installed on your computer. To block a web page can and some of the programs. Check the Windows firewall settings - set a different permission mode, create a rule for connections. If the computer does not have an antivirus, open files can block the virus.
If you are the owner of the site is not available, check the end of the period of prepaid hosting services. The decision to close your site could be taken by the administration server. If entering personal details to your account, you can't get to the site, you may have violated the rules of resource for you and the site became unavailable.
Have you checked all the parameters, the period of paid hosting is you have not ended the rules you violated, then the reason for the unavailability of the site could be a DOS Attack (Distributed Denial of Service).
The number of Internet users is growing as is the number of scams. Originally a DOS attack was used to verify the reliability of communication channels and resilience of the systems. However, this defence strategy has become a problem, which is solved using different methods. Protection against such attacks should be present on any resource, regardless of its magnitude.