After installing some updates to the mysql package, akonadi stopped working and therefore Kmail and Kontact in general would not work. So here's the fix:

Step 1

Edit the /home/your user name here/.local/share/akonadi/mysql.conf file;
(replace 'your user name here' with your actual user name)

Replace the following

with

Step 2

Delete all files in the /home/your user name here/.local/share/akonadi/db_data/akonadi/ folder (once again replace 'your user name here' with your actual one)

Step 3

Open a terminal window and type (NOT as root/superuser):

And you should get akonadi back!