My friend just bought a 2002 Hyundai Elantra and neither of us can figure out how the door locks work. It doesn't have any separate lock/unlock button on the inside of the door like most vehicles have, just the mechanical switches attached to the levers to open the doors. However, the vehicle has power windows, and I would be really surprised if it's lacking power locks. Are the mechanical switches on the front doors (and possibly also the key) supposed to activate power lock/unlock on all doors? If so, aside from the fuse, are there common things that go wrong with this system that we should check out?
Checking with Kelly Blue Book the power window and power locks are listed as separate options. Looking at past reviews some state that the power door locks were standard equipment and are operated by the key fob (remote). The fob is supposed to unlock all the doors according to the original sales information I have found. I would check the fuse panel to see if a fuse is labeled power door locks. If a fuse is in the holder the car most likely came with power locks. With some research you may find a universal key fob remote and program it to the car.
I just got this same car for my daughter. Same thing: power window button, but no separate button to lock/unlock the doors.
However the doors will all sync to the drivers door, at least they're supposed to, using either the key from the outside or the toggle next to the door handle.
In our particular case, all the doors will synch to lock, but they're inconsistent in synching to unlock. However I found that if I can get them all to unlock from the inside of the car if I push the backside of the toggle inwards rather than pulling the front side of the toggle outwards. So I suspect something is wearing out in how it's supposed to work
In your case I suspect a fuse has gone out.
If it has power locks, typically all locks can be unlocked without a remote by unlocking the driver's door with they key twice in quick succession. The locks for the other doors will unlock then if it has power locks.