Your credentials are linked to the email address they were issued to. When you log in, you see every credential issued to that address, and only those. So if one is missing, it has almost always been issued to a different email address.
How to find it
Check your other email addresses. Search each inbox for the credential email, or log out and log in with your other addresses. Use the forgotten password option if you haven't set a password for that address yet.
Check the email you registered with. Your credential goes to the address the issuing organisation has on record, usually the one you used when you signed up for the course.
Ask your issuer. If you still can't find it, the organisation that issued it can tell you which address they used, and can reissue it to the address you prefer. See When to contact your issuer.
Following an old link that no longer works?
Some organisations show credentials on their own web address. If they stop doing that, links using their address stop working, but the credential itself is not lost. Log in at app.certify.one with the email address it was issued to, and you will find it there with a new link you can share.