i have a 1999 ford focus, how do u fix central locking?

my 1999 ford focus wont lock with the central locking you can here the locking noise when you press the key fob but it wont lock, even when you put the key in the drivers door that is the only door that locks and you have to lock the other door by pushing in the buttons and the passanger door does'nt lock at all, can you help?