Car lock problem

Hi,
I have an old Camry 90', with a manual door lock mechanism. One
problem is the driver side door lock cannot open occasionally. Today,
it cannot open anymore. No mattre I press and drag the door handle, or
use the key (the window can be low down). I am in Canada. Somebody
says that it is caused by rust. Is it? How to repair it? Thank you
very much.