I just realized that I didn't include a 1099 form from last year (2015) & I was owed $1k. What can I do?

I withdrew my 401k early to help me pay for grad school. I received 5k on December 22, 2015. I was told that I was eligible to receive money back on taxes (around 1k) because I was using it to further my education. When I filed my taxes in January 2016, the tax preparer told me that there was nothing I could do to receive that money back until 2017. Now, looking at my 1099, the date on it is 2015 & I never got that money back. What can I do?