Fatal error

Our website went down earlier today, and from checking the error log, it appears that an error in WordPress caused it to crash. We rebooted the server and everything works fine now, but I’m just not sure how to fix it so it doesn’t happen again. Here’s the error message:
PHP Fatal error: Maximum execution time of 30 seconds exceeded in C:\\xxxxx\\wp-includes\\wp-db.php on line 560

Here’s lines 559-561 in the wp-db.php file:

function __destruct() {
		return true;
	}

Does anyone know what caused the problem and how to fix it?