This error suggests that you are already logged into Dimensions somewhere, but can also appear if Dimensions wasn't closed properly; due to a power cut or the machine being closed while still logged in.
First of all please ensure that you're not still logged into Dimensions on another machine.
If you are definitely not logged in anywhere, you will need to have a user who can still log in themselves, inactivate your session:
Click on the User Records button (which is under the Security module, which is part of the System tab)
Select the User ID
Click on the Inactive button (which is under the Record module, at the top)
Select OK
Select Yes
Note: a user can still log into Dimensions if the system thinks they are still logged in