You may not have the appropriate permissions to access a file in a folder. If the link is to a shared folder, you may have been removed as a member of the folder or your role may have changed in the folder, limiting your access to the files.
If the folder link was created as a public link, that public link might be removed and therefore the link will stop working. When a public link is sent, a role is assigned to the person receiving the link. If the link was sent with a Viewer role, then files can't be downloaded.
It's possible that someone may have deleted the folder that was shared with you. In that case, the link won't work.