Loan that I paid but now am told it went back to it's original amount even though I had paid almost a 3/4 of it.

In 2013 I took out a loan of r19000 with FNB and my instalment was r1000pm. I paid my loan for over a year and I stopped paying it after I switched banks because there was money constantly missing from my account and after switching banks I notified them and I was told that I need to submit a 3months bank statement reflecting my salary which was highly impossible because I had just switched banks. Currently they have started calling me to pay my loan and I am told that my outstanding amount is r20794,30 and I asked them how that is possible I am told that because I stopped paying my loan, I don't understand why the loan amount went back to it's original amount with interest added how am I to afford it? I would like to know what happened to the money I had paid because it was more than r12000. I would like for them to deduct the amount that I have paid so that I can pay the remaining outstanding loan and I would also like for them to fix my credit bureaus score as this is affecting my life badly I can't even afford to buy my kids a home