This MySQL or variant installation has entered "frozen mode". Maintainer
scripts will avoid making changes or starting the daemon until manually
released from this state. See /usr/share/doc/mysql-common/README for
general information about this mode.

In this particular case, an incompatible downgrade attempt has been
detected. This can be resolved in one of two ways:

1. Change the contents of /var/lib/mysql/ to contain database data that
is compatible with the currently installed MySQL or variant daemon
version. For example: you could restore from a backup. Alternatively you
could do a dump using a future version binary and then a restore using
the current version binary.

2. Switch to a MySQL or variant daemon version that is compatible with
the data currently in /var/lib/mysql/. For example, if you have
attempted a downgrade from mysql-server-5.7 to mysql-server-5.6, you
could "apt install mysql-server-5.7" again.

Please resolve this situation and only then remove the /etc/mysql/FROZEN
symlink. You can then run "dpkg-reconfigure <package>" where <package>
should usually be in the form <variant>-server-<version>.