Call to undefined function mcrypt_module_open() in lib/Varien/Crypt/Mcrypt.php on line 63

This could be caused by a recent upgrade to PHP and you are missing PHP Mcrypt module.

To install it's usually a simple apt-get install php5-mcrypt on Ubuntu/Debian. A Google search should turn up the correct instructions for your PHP and OS version.

http://aryo.lecture.ub.ac.id/easy-install-php-mcrypt-extension-on-ubuntu-linux/

In most of the cases, this issue comes in php updated version i.e. php7 and if mcrypt is not installed -

to install it, follow the below steps:

sudo apt-get update
sudo apt-get install mcrypt php7.0-mcrypt
sudo apt-get upgrade
sudo service apache2 restart