If you are working with Oracle then the problem could be that TNS_ADMIN was not setup in environment variables.
Set the TNS_ADMIN in your windows environment variables, to the path to your tnsnames.ora file and then re-start the server.
Example: TNS_ADMIN:=oracle client/install/product/product_version/Client_1/NETWORK/ADMIN
Steps:
In Windows, right-click 'My Computer' icon and select 'properties'.
Click on the 'Advanced' tab.
Click on 'Environment Variables'.
Under the 'System Variables' pane, search for the TNS_ADMIN environment variable.
Click 'edit' and modify the TNS_ADMIN to include the correct path to the tnsnames.ora file and click 'OK'.