OVER PAYMENT

So a few months ago I received a letter from the IRS stating that my employer has submitted additional wadges for me that I did not claim on my either 2020 or 2021. They were paying me under the table but some how added it to my wadges years later. How is that even possible? :( But also my papers were destroyed so I no longer have them and can't get through when I call the IRS. When I log on to my IRS account it shows no balance. So I am confused on what to do. The IRS is saying that I owe about $7000 for over payment .... Any help would be appreciated because I have no idea what to do!