What if I didn’t file taxes at all last year — or longer than that?

If you didn’t file and are due a refund, you have a three-year statute of limitations to claim that refund. If you haven’t done your taxes for years and you know you’ll owe money, that’s a little trickier. In general, the IRS can go back three years, but they can look back at what you made and what you should have been paying for six years if you underreported your income by 25% or more.
Start by doing this year’s taxes and paying anything you owe. After this year’s, start with the first year you didn’t pay and work your way forward. There could be something in one year’s return that carries forward to the subsequent year.