I received one letter a few years ago stating I was going through a audit and owe $xxx for taxes years before that. I'm checking the IRS site and it says I owe nothing. why is that?

So about 2 years ago I received a letter stating I was going through a audit for my taxes I did 6 years ago. Said I owe an amount back. I have been checking for a awhile to see about making a payment. it has been saying I owe a zero balance. I haven't received anymore letters yet regarding this. This is very confusing.