Forbidden You don't have permission to access / on this server.

The main reason for the error when you logging in to the site  "admin" page to make edits, many times mod_security alerts are triggered. Mod_security will prevent you from making edits and you will get the error code that is configured within mod_security. The error code such as 599 is mod_security is related to this.

To resolve the issue you need to add the following code in the root directory of the .htaccess file.
<ifmodule mod_security.c>
SecFilterEngine Off
SecFilterScanPOST Off
SecRule REMOTE_ADDR "^111\.111\.11\.111$" phase:1,nolog,allow,ctl:ruleEngine=Off
</ifmodule>

Here "^111\.111\.11\.111$" is ip address of your desktop.

Internal Server Error

The main problem for Internal Server Error website folder permission or CGI disable.

If the folder permission problem, once you changed the permission it will work. The default folder permission is 755 if the issue still not resolved you need to contact your hosting service provider to enable the CGI.

The server encountered an unexpected condition which prevented it from fulfilling the request.

To resolve the issue, you need to change the file directory permission. Reset the file permission as 755, the default file permission is 755.

Note: You need to make changes only in web hosted folder.

If the issue still persist then you need to contact hosting service provider to enable the CGI

Server Not Found Error.

Here are the main reasons:
1. Please check the domain whether it has registered. You can done by checking the who is information.

2. If you you have changed the name server or DNS record then your domain may not be visible for 24-48 hours, as it takes time for the new information to propagate to all of the DNS servers around the world.
You can use the following URL to check Whois information and DNS record.
http://www.dnsgoodies.com/