You may need to relink (re-upload) your local files on the following occasions:

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?