A syntax or spelling error, take your time and check the url
The user may have set an expiration date that has been exceeded
The user only allows use from certain IP addresses
The user requires identification information
The user has deleted it again
Other restrictions set up by the user who created it
Last but not least, the administrator may have deleted it. (This could be due to unwanted affiliation with the destination, closure of the user account it is associated with it or for some other reason.)