You may need to relink (re-upload) local files that you previously uploaded in the following situations. Once the relink is complete, all your relevant edits will be restored immediately.
Premium users:
1) Files failed to sync with your Cloud Space successfully;
2) Files have been removed from your Cloud Space.
Free users:
1) Use another browser;
2) Use another computer;
3) Use the New Incognito window;
4) Files have been removed from your computer.
Also read: How to relink missing files?