Thank you for writing to us. First of all, a note of caution: the fact that you have contacted us on a public forum means that your anonymity is likely compromised. If anonymity is important to your situation, please be aware of this before proceeding any further, and please do not provide any potentially-identifying details or followup to us here in this space.
On to the answer: If a codename is not accessible, and you are 100% sure it the right codename (no typos/memorization errors), there are a few possible explanations. A news organization may have deleted source(s) from their database, or made configuration changes to their SecureDrop and not restored their database from a backup, or restored from a backup taken before a given codename was created.
In all cases, the way to contact this news organization via SecureDrop is to create a new codename.
Do not submit or refer to your old codename in your conversation with the newsroom, since they see a completely different name/identifier for you (and since your codename is a passphrase that should always be kept secret).
I hope this is helpful and I wish you the best.
SecureDrop Support team