capturing of wrong detail and failing to give noti

iv been trying to get a detailed report of my account so i can arrange payment but everytime i went to the branch i was told that im on the system, all the while the gym company has been adding to my bill charging me for a service i don't use, i know i signed a contract, however i made an effort to arrange payment due to being unemployed i couldn't pay. Today i went to go check again and to find that my details where captured incorrectly under an id number which doesn't belong to me, and told that im in debt for 3000+ the lady assisting me than told me my account has beezn takin to credit bearu without an sms, letter, or any sort of notification, i havnt changed address or phone number. My issue is how am i to be held in contant for an error thats not mine, the sales agent that put me through captured the wrong details not me, i tried to make an arrangment however couldn't because i was told im not in the system. Please advise urgently!