The issue is with SAGE's login process. If you close SAGE without logging out then SAGE blocks you (or the interface) from logging in. The key is to delete a file called QUEUE.DTA.
To find this file you will need to navigate to your SAGE data folder.
On a Windows machine it can be found:
Note: this assumes you only run one SAGE company, your using a 2013 version of SAGE and that you installed the application in the 'normal' location.
If you cannot find your SAGE database location speak to your IT department or SAGE cover care support.
Once you've found this folder (ACCDATA) find the file called QUEUE.DTA and delete it. Then try running the SAGE interface.