Or don't panic, whatever.
Our hosts over at one.com has pushed all their customers onto PHP 7.2. We were running an older version. Due to this, the site has been displaying a 500 error recently.
I have temporarily rolled us back to PHP 5.2, and I am upgrading the SMF forum software now. After this, I will roll back to the latest stable PHP version and everything should hopefully be hunkydory again.
Otherwise, I would urge you to panic.