Ok the best thing to do would be to first backup the entire site so you have a copy. Then export your database via tools>export from the WordPress dashboard if you’re able to access that panel.
Then on a fresh install (probably best create a new database) and unassign the old one through your cpanel/hosting account.
So now you have a blank site, on the correct domain install WordPress again, then download a fresh copy of the theme from your themeforest account and install via the WordPress admin appearance>themes. Import the database you previously exported from the old site via tools>import from the WP admin area.
Set your menus via appearance>menus, set your widgets and any other menus such as the footer. Via settings>permalinks set to “post name” and save.