I dropped my
mysql deafult system database accidentally.
Reinstallation (as accepted answer suggest) is not always possible, so I tried mysql_install_db first from second answer.
Now it's a part of mysqld and you can try
mysqld --initialize but not on my version of OS and mysql.
Anyway it didn't help me too. So I've read docs carefully and found
If the data directory does already exist, you are performing an upgrade operation (not an install operation) and should run mysql_upgrade, not mysql_install_db
That did the trick. With mysql_upgrade I've restored all system tables.
ps I have to mention, that it was fresh install of mysql server, so I didn't care about other databases, settings or users. Be carefull!!! You can lose important data.