Hi there!
A few things to try
Make sure the Creative Cloud app is open, and file sync is enabled
If your media is on an external drive, make sure it's still connected
If 1 and 2 do not solve this issue, right-click on one of your clips in the timeline, select connect from the menu and navigate to where your media is kept, and re-link it.
Let us know if that helps or not. Thanks!
Peter
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