If another user on your campaign has created a folder but you are unable to see it, the most likely issue is that the folder's sharing settings need to be updated.
The user needs to go into the folder and click "Edit Folder" in the top right:
Then, they will need to find your account under "Users without Access", highlight your name by clicking on it, and then clicking "Add". Then, save the edits.
Try again, and you should be all set! If not, please submit a support request.