WordPress 404 Fix & Update

I’m facing a site-wide 404 error on my WordPress installation. I’ve already tried resetting the permalinks, but every page—homepage included—still throws the error. Because the site runs on both a custom theme and several bespoke plugins, I need a careful, code-safe solution... (Budget: $10 - $30 USD, Jobs: cPanel, HTML, MySQL, PHP, Website Design, Website Management, Website Optimization, WordPress)