Before doing anything drastic, go into PhpMyAdmin and make sure the siteâ€™s content is really gone. Select the MODx database, select the modx_site_content table and click on the browse icon. If you can still see your content there and there are a number of records, itâ€™s not gone, just not showing for some reason. Stop there and let us know if this is the case.
Unless you checked "new Install" (and it shouldnâ€™t have been checked by default unless you deleted some files before upgrading) MODx shouldnâ€™t have trashed your content--and Iâ€™ve never seen it do that.
If the siteâ€™s content is really gone, I would first click on Export (still in PhpMyAdmin) and export the whole thing as a SQL file to your local machine (click on save to file), just in case. You can look through that file with a text editor to double check that your site content is gone.
Iâ€™m assuming that your back up is a fie with a .SQL extension. If not, stop here and let us know what it is.
While still looking at the current DB, you can click on "select all" to put a check mark next to all tables and then use the bottom drop-down "with all" menu to select drop. Confirm and you will have a completely empty MODx database. Now click on import and browse to your backup. Once itâ€™s imported, you should be back where you were when you backed up.
Hope this helps. Ask if you get stuck or confused.