collection problem hurting credit score

Years ago  back in 2012 had a mix up with verizon billing. .Closed and account after I closed the account I was receiving a refund check and a final bill at same time I cashd the check and threw disregarded the bill on some bad advice from a verizon rep.. thought these would offset .to make a long story short  this unpaid portion of final bill was sent  to  collection and damaged my credit can somone please advise on how to resolve issue. thanks