Before doing anything drastic, go into PhpMyAdmin and make sure the site’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, it’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 shouldn’t have been checked by default unless you deleted some files before upgrading) MODx shouldn’t have trashed your content--and I’ve never seen it do that.
If the site’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.
I’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 it’s imported, you should be back where you were when you backed up.
Hope this helps. Ask if you get stuck or confused.