What the IRS Does
The IRS files Substitute for Returns using the worst possible assumptions: single status, no dependents, standard deduction only, no credits. The result is always a higher tax bill than what you would actually owe. Then they assess that inflated amount and start collecting.
How to Fix It
Pull IRS transcripts to see what the IRS already has. Prepare accurate returns for the required years (usually the last six). File them and watch the balances drop as your real deductions and credits are applied. Then deal with whatever balance remains using installment agreements, OIC, or CNC status.
Criminal Risk
Willful failure to file is a federal misdemeanor. Filing voluntarily before the IRS contacts you dramatically reduces criminal exposure. The IRS rarely prosecutes taxpayers who come forward on their own.
The hardest part of fixing unfiled returns is starting. After that, each one gets easier and the weight gets lighter.