it is possible that after upgrading the plugin the Lightbox gallery style (defualt) stops working.
Also, any updates to the WP's theme can prevent the plugin from working correctly with a specific gallery style.
Issues are rare, but can happen.
Please try the following:
- verify you are running the latest version of the plugin
- post here a page where you inserted a gallery that you would expect to see working. That way, I can check what's wrong.
- Try installing a different galery style by choosing from http://www.simplestgallery.com/add-ons: the lighbox style might have issues on your theme, but others may work.
Hope it helps