Bank admits they made a mistake but I am still out $965. What can I do?

I found an old check from my old employer. It was for $965. My friend said it was real, my father looked at it and said it was real too. I couldn't believe I had forgotten about a check! Who does that? It was dated right around the time that I changed jobs so, the only explanation I could think of was that it must have been the last check from my last job and I just forgot about it because I was already working elswhere. I took it to my bank to ask them if it was a valid, cashable check. They said yes and cashed it for me. The unexpected windfall of almost a thousand dollars went fast. I spent it all. Then 3 days later, the bank calls me and says they made a mistake and that due to human error, they didn't notice the check was non-negotiable and they are taking the money out of my account and that they are so sorry. The teller who cashed it called me and told me she'd get fired if I didn't give the money back. I still said no. What can I do to get my money back?