It could happen after you update your php version, for instance if you upgrade from php5.6 to php7.1 you need to run these commands:
sudo apt-get install php7.1-mbstring
sudo service apache2 restart
If your destination version is different you need to check if the mbstring package exsit or not, an example for php7.0:
sudo apt-cache search php7.0-mbstring
I found it useful to first check existence of all modules that you working with, then performing an upgrade, in addition to that update phpmyadmin after upgrading your php is a good idea