My folders and messages are no longer there. All my folders are gone!
It is likely that you are logged in with a different account. For instance, you may have sent a test message to another email address of yours and then accessed that message. In the process you may have logged in to the other account.
Please check which account you are logged in with: look at the email address displayed near the top right. You can click on that address to change the account you are logged in with. If on a mobile device, you may first have to click the menu icon in the top right and then find the email address in the drop-down menu.