If you cannot access your website, but do not have redirect loop errors, your SSL certificate might be invalid. Normally Really Simple SSL checks if ssl is available, and only proceeds if the test is successfull. If you forced ssl anyway, or blocked access to the plugin folder in an unexpected way, you could accidentally have installed your site to work on SSL, which results in browsers blocking access to your site.
If you get redirect loop error messages, look in the knowledge base category “redirect loops”
For other error messages: when your SSL certificate is invalid you might get one of the following messages:
Cannot connect: Could not connect to server
Safari Can’t Connect to the Server
You probably do not have a valid certificate. First, uninstall the Really Simple SSL plugin, following these instructions.
Your second step should be to contact your hosting company, and verify that the SSL certificate they installed is valid.