The IRS has an entire division whose purpose is to investigate individuals and businesses for criminal tax violations – including non-filing, non-payment, unreported income, false deductions, etc. These crimes can be felonies and can involve prison sentences.
Interestingly, many misdemeanor cases are elevated to felonies due to subsequent acts by the taxpayer, such as making false statements to an IRS agent.
If you have any risk of criminal prosecution, it is absolutely essential that you contact an experienced federal tax attorney who specializes in criminal tax cases. And do so as soon as possible while you still have choices.