It could be a number of things without seeing the logs, possibly plugins or themes. The .htaccess that Tim provided is a file in the root of the WordPress install. It's notorious to cause 500 errors for WordPress installs. You may try to look at this file by asking your host on how to gain access to your site's files; majority of the time it is via FTP.
I would also try to reach out to your hosting provider for your site's error log. Once we can see the error in the error log it makes troubleshooting much easier to identify the issue.