You can check for two scenarios:
1) Your project never fully finished syncing to Creative Cloud --> check for a blue circle link in the upper right corner of the project, next to the undo/redo arrows. Make sure it goes blue ALL THE WAY around, indicating that it's syncing; it'll disappear when all assets are uploaded and ready to use in the project.
2) You may not have deleted your project assets, but if you MOVED them before the project fully synced, that would definitely cause the missing assets indicators that you've shown. RIGHT/CONTROL Click on one of your clips, and look for the "Find & Connect" to point to your files' new locations.
Hope this helps!
Anissa • @anissat
Hope this helps! Make sure to press "✔ Correct Answer" on this post if this answers your question. Happy Rushing!