This sometimes happens when one or more users exits Dimensions incorrectly, or leaves themselves logged in long enough to time out.
If someone is already logged in, to force other people to log out:
Click System, Security, User Records
Locate and select the user you want to force out.
Click Inactive
Read and agree to the first prompt, and then click Yes to log them out.
If no one is already logged in or able to login, our support team will need to investigate, run scripts or liaise with the hosting or development teams. Any Issues with contacting us, please see the following Get support in Dimensions.