If you ever use Site Aggregator Web Part on the
default.aspx page of a site and configure it in a way that redirects users to some other website (specified when adding Site Aggregator) every time they try to open the SharePoint site (I have no idea how I managed to do it), you can use the
Web Parts Maintenance Page to resolve this problem. Of course, you can use this page to troubleshoot all other kinds of problems caused by Web Parts misconfiguration.
The URL of the Web Parts Maintenance Page is:
http://SITE-URL/_layouts/spcontnt.aspx?&url=%2fPAGE-WITH-WEB-PARTS.aspx