Upgrade from 16.04 LTS to 18.04.1 LTS - MySQL server problem

[FrancescoHEHE]

Hello,

I upgraded from 16.04 to 18.04.1 with no apparent problems (hosted on a 1&1 dedicated server)
This server hosts a number of Joomla!-powered websites.
Had to restart MySQL server, and then there was no more response from MySQL server.

Have tried re-installation with :

Code:

sudo apt-get remove mysql-server
sudo apt-get install mysql-server

Unfortunately (ie none of our sites are running now)
I tried stopping and restarting the MySQL server with no success.

Tried re-configuration with :

Code:

sudo dpkg-reconfigure mysql-server-5.7

Got this message after re-configuration :

Code:

Checking if update is needed.
This installation of MySQL is already upgraded to 5.7.35, use --force if you still need to run mysql_upgrade

It also didn't help... I'm not too familiar with MySQL server (I've installed it several times and never had a problem... I've used it without incident) but now I'm at a loss!

Has anyone got any suggestions for what I should do?
If you require any additional information, please do not hesitate to ask (please dive me the bash command to enter, as I am not fluent in Bash?)

Thank you in advance for your assistance and advice.