It is virtually impossible for someone to steal your password unless you have been careless in choosing or guarding it. If you are simply having trouble accessing your Web site, be sure that you are entering your user name and password correctly, including case sensitivity.
Incorrect case is the most common cause of password problems.
If you have exhausted the other possibilities and someone actually has obtained your user name and password, then you need to do what you can to find out who has accessed your site and negotiate its release with them. If you do not know who it was, you may want to examine any content changes for clues of authorship. It may be that your best option is to take legal action against them personally or persuade them to return control of your site. As a last option, you can sign up for a new site if you cannot regain control of your stolen site.