Cause

After reinstall mariadb, replace all DB files in folder “data” directly could cause this error and service “MariaDB” will never started.

Solution

1. Check error log:
   https://mariadb.com/kb/en/library/what-to-do-if-mariadb-doesnt-start/
   C:\Program Files\MariaDB 10.4\data\REX-PC.err
2. Create an option file and put it in %WINDIR%/my.ini
   https://dev.mysql.com/doc/refman/8.0/en/option-files.html

   [mysqld]
   innodb_force_recovery = 6

   1. Why 6?
      https://dev.mysql.com/doc/refman/8.0/en/forcing-innodb-recovery.html
      https://mariadb.com/kb/en/library/innodb-recovery-modes/
3. Run command to start mariadb: (under folder bin)

   mysqld --verbose

4. You should be able to connect to DB server now
5. Export tables in standard way.
6. Uninstall DB server
7. Install DB server