Turning on URL Rewriting broke my site

Why it happens

With URL rewriting on, every address goes to index.php through a server rule. If the rule isn't there or doesn't fit your server, pages give 404 (rule missing) or 500 (rule not allowed or wrong).

Fix it

1. Check the file is called exactly .htaccess (with the dot, not .htaccess.txt) and sits in the Joomla folder.
2. Apache needs mod_rewrite enabled and AllowOverride set so .htaccess is read. Ask your host if unsure.
3. If Joomla is in a subfolder, open .htaccess, uncomment RewriteBase and set it to the folder, for example RewriteBase /mysite.
4. A 500 right after renaming usually means your host doesn't allow some lines (for example Options +FollowSymLinks). Comment them out one by one with #.
5. On Nginx, add the try_files rule instead; .htaccess does nothing there.
6. Need your site back now? Set $sef_rewrite = false (or 0) in configuration.php.