If this is the first notice you have received, the notice should tell you why you owe the additional money, and what to do if you disagree. Read the entire notice carefully.
What is the notice number or letter number at the top right, starting with CP or LTR? Is it about your 2019 tax return?
You haven't given any details, but I'll take a guess at what might be going on. If you owed such a large amount when you filed your tax return, the IRS might be billing you for penalties and interest for late payment of the amount that you owed. You should have been making payments throughout the year, by withholding or estimated tax payments, instead of waiting to pay it all when you filed your tax return.
