In some cases power failures will cause Sims to drop its link to the database (On Standalone Setup).  Follow the next procedure

1.  Double click on Sims. Click on the arrow. 
2.  A window will appear with a heading "Please choose datasims directory..."
3.  If datasims is on the local C:drive it will automatically open in the folder.
4.  Click on Open.  The blue progress bar will start running on the bottom of the screen.
5.  When the procedure is done the login screen will appear.