This should fix it:
Open Safari > click on Safari menu > Preferences > Security > Internet plug-in's: Plug-in Settings > Allow Microsoft Silverlight plug in
Or:
Launch Safari > Click Safari in the top menu, then Preferences > Click Security and put a tick next to Allow Plug-ins > Click Manage website settings, make sure Allow is set for Silverlight and click Done > Click Advanced then remove the tick from Stop plug-ins to save power.