Recovering MySQL 5.7 tables in MariaDB 10
Before doing anything else I would recommend that you backup the old mysql data directory and the configuration file on /etc. So should something go wrong you don't loose your data.

Then I would check what specific MariaDB version of you have installed. As per MariaDB compatibility chart, you need MariaDB 10.2 to migrate from MySQL 5.7. Once you are sure you have the right version installed, you should refer to the migration doc:

https://mariadb.com/kb/en/library/migrating-to-mariadb/

Alternatively you could uninstall MariaDB and install MySQL again, just check that you are installing the latest 5.7 version available. For this you can download a deb package from the mysql downloads and install it manually or set up the mysql repository: https://dev.mysql.com/downloads/repo/apt/