If potentially useful evidence is controlled by someone you no longer speak to, document the situation instead of assuming the case is ruined or taking unsafe steps to obtain it.
First identify exactly what is missing
Write down what you believe the person has, what fact it might support, why you think the evidence exists, and whether another source could document the same fact. Separate “I know this document exists” from “I think this person may have something useful.”
Add the item to your missing-evidence log.
Look for independent alternatives
Depending on the fact, alternatives might include your own messages, cloud backups, photographs, medical or police records, travel records, public records, other witnesses, receipts, emails, social-media archives, or country-condition material. One unavailable document does not mean there is no other corroboration.
Do not create a safety problem
If contacting the person could expose you, the other person, family members, or witnesses to danger, tell your attorney. Do not pressure, threaten, impersonate someone, access an account without permission, or ask another person to obtain evidence unlawfully.
If you do make a safe and lawful request, preserve the request and response. If there is no response, keep the record showing when you tried.
Prepare an attorney note
Record: the missing item; who controls it; your relationship to that person; why contact ended; whether contact is unsafe or simply unavailable; any attempts to obtain the item; and possible substitute evidence. This gives counsel facts to evaluate rather than a vague statement that “I cannot get the evidence.”
If the issue involves a deadline, raise it immediately. Immigration-court filing deadlines can affect whether later evidence is considered.
Official procedural resource
This is general educational information. A qualified attorney or accredited representative can advise how missing corroboration affects a particular case.