You need to deactivate s2Member. To do that (as with any WordPress plugin) use an FTP client like FileZilla or your hosts's File Manager and rename the s2Member folder (e.g. call it
Now you should be able to get in to your site and work out why you were locked out. I'd start by checking whether you have protected some pages that should not be protected, or whether another plugin (especially a security or caching plugin) is causing the problem.