If you've lost access that you had before, here are some things to check:
Software updates could cause tabs to have been moved.
If you are an employee, you can check with your "Administrator" if he has made any changes to the access profiles or the subscription of the center.
If you are the administrator, perhaps you have made changes to the chosen modules. You can check in the administration, under "Billing".
If you have late payments, your subscription may have been temporarily suspended. In this case, you will have been warned in advance.