First of all, please ensure that you are not logged into Dimensions on any other computer as this error can be a genuine warning to let you know that you are still logged in elsewhere.
If you are sure you are not logged in elsewhere, then the error is likely to have occurred because Dimensions wasn't closed properly; due to a power cut or the machine being closed while still logged in.
To resolve the issue, you will need to have a user who can log into Dimensions make your session inactive. As this requires access to the system control and user areas of the software, this will usually be your hosting.admin user or a CLOUD\ account with administrative permissions:
Within Dimensions, click on System, User Records
Highlight the relevant user in the list
Click on the Inactive button
Click OK on the first warning message
Click Yes on the second warning message
You should now be able to log in as normal
You can also be granted Master Status on Dimensions which means you can still log into the database if there has been any connection issues.
In the main menu, click System.
Click User Records.
Find the user, click Options.
Click Edit.
Select the Master Status check box.
Click Save.