Site wont let me into administator areas Whenever I want to go to an admin only area (e.g. Modules), I get this error:

Fatal error: require_once(): Failed opening required
'/Applications/XAMPP/xamppfiles/htdocs/Ecommerce_website/sites/all/modules/openatrium-7.x-2.33/modules/system/system.admin.inc'
(include_path='.:/Applications/XAMPP/xamppfiles/lib/php') in
/Applications/XAMPP/xamppfiles/htdocs/Ecommerce_website/includes/menu.inc
on line 517

Any ideas?
(Tip: I have also tried putting openatrium back into the folder and it still gave me an error)
Why has it happened?

OpenAtrium is not a module, is an installation profile

You are mistaken an installation profile with a module. An installation profile contains the whole drupal core plus some extra modules and themes.

It means, you cannot put it in the modules directory of another Drupal installation. If you want to use an installation profile you do not need to use Drupal core.

How to fix it?

A: Re-install Drupal using OpenAtrium

If you want all the features in Open Atrium and you do not care about your current website, you'd better delete the whole thing and re-install Drupal using OpenAtrium.

Extract the content of OpenAtrium package to your document root (which is /Applications/XAMPP/xamppfiles/htdocs/ in your case). Then install it again.

B: Remove OpenAtrium and keep the current installation

If you already have a working Drupal installation and want to keep it, first you will need to remove OpenAtrium package out of it. There is no way you could use an installation profile along side a current installation.

First remove all the OpenAtrium files.

Then clear all caches. You can go to database and clear the content of all tables starting with cache_.

To access database you can use phpMyAdmin in browser (learn how) or an app like SequelPRO.

If the problem persists, you will need to rebuild the menu.