Hijacking the top comment because this just happened to me, and I hope you end up reading this. Being firm, like this guy says, could come across as angry, and that does not help! I will tell you what I did to get the answer, and the money back into my account.
My wife works at an answering service, and she takes calls like this all the time. There's a protocol that they are obligated to follow, or they lose their job. Most of the time they have no idea what's going on, and are just there to get you to somebody that does. More than likely you're going to be talking to a teller, or an answering service to start.
They have no idea who you are, or what is going on, and may not even have access to that information. So be really nice to them, because they are taking your message, and passing it on to their supervisor, and are going to either say something like, "they were really upset and insisted that they reach you," or "they sounded really concerned, and need to figure out what's going on." Nobody wants to deal with somebody who is really upset, and so you're more likely to get the runaround in that case.
Knowing this, the first thing I did was wait until I calmed down a little. Seeing the money suddenly disappear from the account like that was pretty frightening, and I wanted to immediately pick up the phone and demand answers. Instead, I went in to the kitchen, heated up some water, and made a nice, strong cup of coffee. I made sure my bladder was empty, and my stomach was full, and went back into my office.
I then got out a notepad and pen, made sure it worked, and called the bank. I got through the automated system by saying "agent" over and over again. When somebody finally came on the phone, I politely, with a smile, even though I was still pretty upset, and could have lost it at any moment, explained the situation. I told them that I got an alert on my phone that a large withdrawal had been made, and I did not authorize it, and I was wondering if they could tell me where the money went.
They told me that it was a legal hold on the account, which was even more frightening. Still, I kept my cool and asked them who it was that put the hold on the funds, and how I could get hold of them. Now here's where things got really frustrating. They told me that they didn't have that information! Holding back my frustration, I asked, with a smile, "Is there anybody I can speak to that might have that information?"
I got transferred a few times, and eventually had to call their legal department. After about 2 hours on the phone, I finally got the number to call of the people that put the hold on the funds.
Long story short, it turned out that 5 years ago I made a mistake on my taxes and didn't pay enough. I moved a couple of times in the past year and they didn't have a current address or phone number to get in touch with me so from their perspective I had been ignoring their letters and phone calls, and they decided it was time to collect. I gave them my new address and phone number, and explained calmly to the agent on the phone that they took out more than I could afford, which was going to be a hardship (make sure to use that word) for me, and asked if I could make payments instead of paying the full amount up front. The agent asked me how much I needed, and I told him.
In the end he ended up releasing the funds for twice as much as I said I needed, and instructed me on how to make the payments on their website.
Every person I spoke with thanked me for being patient and polite. I don't think I would have gotten anything back from them had I not been as polite as possible. I owed the money, and they had no obligation to give any of it back at all. If I had just called them up demanding answers, all I would have gotten out of it was a huge headache.
TLDR: Be nice, you're dealing with people whose jobs depend on following a certain protocol. They want to give you the answers, you just have to be patient, and realize that they do not have as much control over the situation as you might think.