Upon upgrading to eM Client 8, all your folders were “hidden” by default under a folder called 'More'. This was to give you a fresh start with your folder management and make the interface more streamlined.

To remove items from the More folder, simply drag them out of it. Once there's nothing in the folder, it will disappear. To add items back, simply right-click an item and select Hide.

Alternatively, you can manage which items are hidden under the More folder or shown on your folder list by right-clicking your email account name in the folder list and selecting Show/hide folders.