nissan altima 2003 - back seat belt stuck - dania

when I brought my car the seat belt worked fine but lately was getting stuck more and more often unti yesterday it will just wont come out I tried everything but failed I saw a key lock in the middle of the seats but I have no idea what is for I tried to unlock it with the car key but I doesnt fit can someone else tell me how can I fix this am I the only one with this problem?