unpaid account

Good day, i am so disappointed about how my account was handled, last year May i settled my account with Nimblegroup, and was advised that a settlement letter was gonna be send to me and that they will send prove of payment over to the store, till today this has not happened, i called the store to find out if they had received the letter from Nimble to advised that the account was paid up, they advised me that the account was never settled. i am ****en angry, i tried calling Nimble since last week i cant get through, even the whatsapp chat does not work, how will i resolve this problem, i am still listed on the credit bureau thats how i saw the account is still active. What service are they providing to they clients, if they ****en cant even do they jobs.