Bunch of Bullies

I have a loan account which was under debt review then I came out of debt review last year June and I continued paying for the account but then they immediately decided to sell my account in August even though I was paying for it and I'm still willing to settle the arrears amount in full. They said my account was with MBD but the last I could get hold of someone there they advised that they didn't have the account, so now I'm confused on what to do as this is really hurting my credit profile. I would like to pay for my arrears and get my account back on track as soon as possible please. After paying arrears I can then