I received a letter from the IRS stating I did not claim all my income last year, which I used Turbo Tax, and now the IRS says I owe them 20k by March 16th.

I borrowed money from 401K last year and showed it on my taxes. The IRS states, it was not reported and I owe the IRS 20K by March 16th. Why is the IRS stating the money was not reported?