How can I restore the MySQL root user’s full privileges?

If the GRANT ALL doesn’t work, try:

1. Stop mysqld and restart it with the --skip-grant-tables option.
2. Connect to the mysqld server with just: mysql (i.e. no -p option, and username may not be required).

3. Issue the following commands in the mysql client:

   UPDATE mysql.user SET Grant_priv='Y', Super_priv='Y' WHERE User="root";

   FLUSH PRIVILEGES;

After that, you should be able to run GRANT ALL ON *.* TO 'root'@'localhost'; and have it work.