Rep Power: 0

I have a 2004 Civic sedan that the power door lock on the drivers side front will not lock. I can't even manually push it down. All other doors lock when I press the button on the FOB. What would be the cause of this?

 
Rep Power: 518

Usual suspect is a bad power lock actuator for the drivers door latch.
Easily checked with a test light or a DVOM and a little work.

-------

With the drivers door open: You MUST pull either the inner or outer handle out before you can push (or pull) the lock button. It's a Honda thing. Simple once you get used to it.

With the drivers door closed: If you still can't manually push the lock button, you may have a latch or linkage problem.

 
Rep Power: 0

Never had to replace an actuator before. Is if fairly hard as a DIY or should I have a mechanic look at it? If not, know any good YouTube videos for help?

 
Rep Power: 518

Figure out (test/diagnose) if it really is the culprit before trying to take it out of the door. (I'd hate to tell you to change it only to have you get mad because that didn't work and it was something else.)

It's attached to the door latch. You have to remove the door latch. Got a service manual?

It's DIY for me, but nearly everything mechanical on a car is DIY for me. I can't evaluate your abilities or tools.