The IRS will contact you and ask for back taxes with accrued interest. While you are emailing or calling back and forth the interest will build up. I experienced this myself when the IRS made a mistake and the back and forth kept interest accruing. I ended up just paying because the cost to hire an attorney would exceed the small amount the IRS claimed that I owed.
You can work out a payment plan with the IRS or they will put a lien on your salary. As a businessman I sometimes got contacted by the IRS to put a lien on someone’s salary who owed back taxes.
In the case of large amounts of money and deliberate fraud, you will be taken to trail and serve prison time if found guilty.