Central locking

I know this has been covered before, but I cant seem to find the post. Heres whats wrong.......It doesnt work!!! The passenger side door doesnt lock when you use the key on the drivers side, nor can it be opened by using the handle on the inside of the door. You need to use the 'emergency' handle on the floor and my girlfriend reckons its going to snap one of her nails (ahhhh bless!). The little motor thing whirrs like it is working but doesnt do anything! Ideas....costs etc (can do the work myself when I get the energy!)