Recieved Judgment but original creditor not listed on credit

I received a summons for a debt that apparently was accrued while I was in college. When I got the summons I assumed that my roommate had possibly stolen my credit card (along with other things) but I have no record of the account.
I can't find anything with that account number listed on it. According to the summons from LVNV, the debt is originally with Citibank but it is not listed on my credit report. First, I called Citibank to ask about the account with the number they listed and they said they have no record of it. When I asked for a validation from LVNV back in December, they just sent me a copy of what they had already sent me. It's just a printout that shows what I owe them and that the account was originally opened with Citibank in 2004.
I know I need to show up for the court date but what do I do if I have no proof that this account with my full name associated with it never belonged to me?
LVNV shows up on my credit report but Citibank does not nor did it show up in 2008 when I purchased my home. Wouldn't that Citibank account be on my credit report in 2008 if the account was opened in 2004? It also shows that Weltman, Weinburg, Reis reviewed my credit history last year for collection purposes. I'm so confused!