Always backup your MySQL database with mysqldump but if you didn’t you can restore it from file system.
If your backup include the /var/lib/mysql directory, if so then you should be able to restore the database. Each database is contained in a separate subdirectory off /var/lib/mysql, so look for one named with the name of your database (WordPress or any other). To restore it, shutdown the mysql server first
sudo /etc/init.d/mysql stop
Copy the backup-ed directory into the same location on your new server, then restart mysql server
sudo /etc/init.d/mysql start
Note:- that its much better to take actual database dumps using the mysqldump command than to rely on this method. so for the future, you should schedule something like this:
If you have to or want to change the domain of your WordPress MU site. The domain name is actually stored in different places in the database, as well as online in wp-config.php. Every instance of these will need to be changed.
Before you do anything create a backup. Go to phpMyAdmin, select your database, and choose Export. Save it to your computer. We’ll be making a copy of this and working on the copy. This way, if anything goes wrong, we still have our backup.