I need help troubleshooting a PHPMailer error

I have a contact form that's been working just fine for quite some time. Suddenly, emails stopped sending and I'm getting a "Email has not sent because of following error: Could not instantiate mail function" error.

I need help troubleshooting. I can provide you specific files, but cannot provide you the entire site to install... so you must help me troubleshoot (I'm a front-end developer myself).