sudo tail /var/log/mysqld.log

MySQL Daemon failed to start.
Starting mysqld:. [FAILED]

If you get an error like "I'm not sure what to do...", you can look at mysqld.log.

But I couldn't figure it out.
The following site solved the problem.
(at sentence-end, falling tone) indicates a confident conclusion
chown -R mysql:mysql /var/lib/mysql
Once this is done, it starts up in one shot!

Mysql startup has arrived!