In most cases, resetting Pinnacle Studio resolves the missing contents issue.  If resetting Pinnacle Studio does not help, re-installing the "Standard Content" will resolve it.

Please follow the steps outlined below:

• Press Windows + R keys together to get Run command box.
• In the Open box, type (or copy and paste) %localappdata% and press enter key.
• Rename the Pinnacle_Studio_24 folder to Pinnacle_Studio_24.old
• Launch Pinnacle Studio 24 as administrator by right clicking on Pinnacle Studio 24 icon then select "Run as administrator" and test

NOTE: Above steps will reset all your Library settings to default, including Watchfolders and Project Bins, and any media previously imported will need to be imported again.

If problem persists, try the steps below to reinstall Standard Content

• Exit Pinnacle Studio
• In your Pinnacle Studio 24 installation folder, you will see 64bit>StandardContent. 
• In that folder, look for the file StandardContent.msi. 
• Right-click on it and choose Install.

NOTE: By default, Pinnacle Studio 24 installation folder (PinnacleStudio24) gets created on your desktop during the installation.