Nucleus has a backup/restore option that super-admins can use to create a backup of the database. It's strongly encouraged to take a backup regularly (weekly or so). The backup-files that are returned are files containing standard SQL-queries, that reconstruct the state of the database as it was when the backup was created.
While backing up is easy, and restoring should also be easy, problems might pop up when your database is fucked up beyond repair. In that case, the repair function might become unaccessible. Below are some ways you can restore your database in that case:
If you have a web-based interface through which you can manage your database (e.g. PHPMyAdmin), there's most likely an option where you can import a file into the database. Use this function to restore your database.
If you have a shell account, restoring a backup can be done by running the mysql program with the following arguments:
mysql -u username -p -h hostname databasename < backupfile.sql