Does your host have something like phpmyadmin in their control panel? If yes, you can use that to create a backup of your database.
Just go into phpmyadmin, click on the database you created to install MODx, and then click Export. Here’s an article that my help you with that:
http://fragments.turtlemeat.com/mysql-database-backup-restore-phpmyadmin.php
Alternatively, if there’s no phpmyadmin, you need to find out what database management tool your host has installed on their control panel. You can always ask your host to create the database backup for you, just open a support ticket with them.
The other thing you want to backup is your site files just in case you need to restore them. You can do this using an FTP client like FireFTP on firefox, cuteFTP, winSCP, etc. Just google them. Your host should have sent you your FTP details when you created an account with them.
If you’re having a problem with your MODx install, one option to try before you do a complete reinstall is to install MODx in "upgrade" mode. This will preserve all your site content. Reupload the MODx files to your server just like you did the first time. Then navigate to the install folder. http://whateveryourdomainnameis/install
Follow the steps but make sure you select "Upgrade Existing Install" and then make sure that you DON’T select to install the sample site. This should restore MODx without erasing or interfering with your current website. But make sure you do backup your database and files as I described above before reuploading and upgrading.
Hope this helps!