Mysql 5.7 cannot start again
The error log of MySQL records the details why MySQL can not restart, check the error log and update it in Questions would be helpful.

If MySQL is installed by apt-get or some other system's ways and uses the default configurations, error log always in /var/log/mysql.

If my.cnf changes, check the modified my.cnf in /etc/mysql/my.cnf or other paths to find the log_error param.