Two situations that look alike and are not
In one, you sent the Zelle transfer yourself, after a call that appeared to come from your bank's own number warning about suspicious activity, or a deposit for an apartment, or a seller on a marketplace, or someone you had been speaking with for months. In the other, you did not send it at all: someone reached your account or your phone number and moved funds out. Banks treat these two categories quite differently, and the difference is the single most important fact in your report. People often blur the two while describing events quickly in a stressful call, and a description given once is difficult to unwind afterward.
What to report and record right away
Contact the bank immediately and follow the phone call with a written claim, keeping a copy of what you submitted and the claim number. Then preserve the detail: transfer confirmations with their timestamps, the recipient's phone number or email exactly as it appeared, the full message thread, call logs, the listing or profile, and any screenshots taken before the other side deletes the account. Describe what happened in plain sequence and only what you know. Accuracy in that description matters more than speed, because the wording is what routes the claim.
After the claim is filed
Banks review these under their own procedures, and outcomes differ considerably depending on which of the two situations above applies. If a claim is declined, ask in writing for the reason given and for the records the decision relied on. Complaints can also be made to banking regulators and to state authorities, which is a separate channel from the claim itself and does not replace it. Beyond that, there may be claims against whoever received the funds if they can be identified, and sometimes against others in the chain who moved the money along. Whether funds can be traced depends on where they went and how quickly. Bring what the bank told you and what the records show, and we will say plainly which parts of this are worth pursuing and which are not.