I’ve had that happen. It’s not hard to fix... google the debt collection agency and find a phone number. You can call and ask for all the info you need. (You’ll need to know your social security number.) They can tell you where it’s from, the balance, and interest rate and while you’re on the phone with them you can set up a payment plan. I’d make sure to develop a budget before I’d call... list my income (estimated if need be), and money needed for food, rent, and such. So you know what you can afford to put towards paying off your debt. 

IF you get your full credit reports from www.annualcreditreport.com, the lender or creditor will have contact information associated with it, you can call or write a letter.

If you can make a large lump sum payment, you may be able to offer a compromise; the collections agency may be willing to accept a lump sum of 75%-80% or so of the amount owed, if you can pay it right away, instead of taking 100% as small payments over a long period of time.