For tax year 2005 I received a 1099 from a payer for the amount of $5025.00. I used this figure to do my taxes.

I have just received an assessment from the IRS stating that I owe $8888.76 in back taxes and penalties. The discrepancy is based on a 1099 figure in the amount of $28283.00 from the same vendor. I never received a 1099 in that amount from them.

How should I proceed?

Thank you.