1. Stop MySQL service. Run services.msc as administrator and stop it.
2. Run cmd.exe as administrator.
4. Run another cmd.exe as administrator.
6. Execute query:
UPDATE mysql.user SET Grant_priv='Y', Super_priv='Y' WHERE USER='root';
7. Close cmd windows.
8. Kill mysqld process from processes list in task manager.
9. Start MySQL service. Run services.msc as administrator and start it.
10. Open phpmyadmin, login and execute:
GRANT ALL ON *.* TO 'root'@'localhost';