Killing a process without login to MySQL
For cases like this, there is "one extra connection available".  Anyone with SUPER priv (eg. "root") can get in to kill the DROP, which would have uncorked the mess.

However, a prerequisite for doing that is to not have users and applications log in as "root".  Instead they should have their own logins, preferable (for security reasons) limited to the database that they need access to.

Reserve "root" for 'admin' actions such as the one you needed in this case.