Oracle MySQL, left as MySQL. Not converted in the middle of an outage.
Will not start, auth plugin rejected the app, or the disk filled the datadir. Hestia and DirectAdmin are usually MariaDB. If this box is MySQL, it stays MySQL.
The auth plugin, the version, the package
An older PHP or a connector that still expects mysql_native_password. We change the account plugin if that is the mismatch. We do not downgrade the server to suit one script.
Disk full, a bad upgrade, InnoDB refusing recovery. Space is freed first. A force-recovery is a last step, after you know there is a dump.
WHM may be running MySQL or MariaDB. The package decides the job. We do not swap them because a guide preferred the other name.
If the binary is mariadbd, this is the wrong page
The symptoms overlap. The package does not. A conversion from MySQL to MariaDB is a planned migration with a dump, not a troubleshooting step.
€55 an hour. Down is emergency, subject to availability.
Minimum 1 hour. Emergency €95. A version upgrade waits on a dump and a note of which PHP connector the sites use.
MySQL rejected the app, or will not start?
Tell us the version from the error log, not the password. No root password in the first message.