Restore Joomla Site 500 Error

My Joomla website has suddenly gone offline and is now returning an HTTP 500 Internal Server Error. It sits on a self-managed Linux server and, to the best of my knowledge, nothing was recently changed—no updates, no new extensions, nothing... (Budget: $10 - $30 AUD, Jobs: Apache, Content Management System (CMS), Joomla, Linux, MySQL, PHP, System Administration, Troubleshooting, Web Hosting, Website Management)