[Plugin: WP-SimpleViewer] Gallery not showing

Are you upgrading galleries from an older version of the Plugin? If you are upgrading galleries from v1.5.4 or older you need to check here: https://www.simpleviewer.net/simpleviewer/support/wp-simpleviewer/#v1

Does your server fullfil the installation requirements? https://www.simpleviewer.net/simpleviewer/support/wp-simpleviewer/#req

Please try this:
1) Install latest version of the plugin (currently v2.1.3).
2) Create a new gallery