Sallie Mae has been giving me problems

Sallie Mae has been giving me problems for the longest time. I have been paying a student loan amounting to $28,000.00 for almost 20 years and have repaid all the money that I borrowed. They informed me that I still owe them $32,000.00. I have spent so many hours talking to them and sending them letters and nothing has been done on my case. I received so many calls from them demanding that I pay my remaining balance. Their calls comes at an average of 7 calls per day, 7 days a week. Just recently, I checked on my Sallie Mae’s account online but I found out I can no longer log in. I don’t have any way of tracking of what they’re doing to my account. I hope someone can help me.