How I received a letter stating that it was reported to them that I did not claim all of my income? All income was claimed, how do I correct this?

The income that they say that I did not report was for my 2017 taxes. They claim that they received notice that I was paid income from SS/Railroad Retirement, and they have adjusted the tax amount for that year and now state that I owe them $6,441 plus interest by 7/17.  I have never worked for the railroad, nor am I retired.  I am however on SSDI, but these wages have been filed on my returns every year.  How can I correct this?