Decades ago I was in that boat myself one year: I became very ill a few weeks ahead of tax day and was hospitalized through it until a couple weeks after. This resulted in totally spacing on filing my taxes. The next year I couldn't find my prior year paperwork but thought I just misplaced it and just filed that year normally.
The next year after that I got a letter from the IRS "we computed your taxes for you, you owe a million dollars"-- quite a heart stopper!
In reality I owed something like $1k, and much of that was just the penalty for failing to file.