My keyless remote functionality packed it in a couple years ago, so lately I have been using the key to manually lock and unlock the door. Yesterday I opened the door and the usual key resistance was gone. Now I cannot lock the car at all. Any ideas on what has gone and part suggestions?

 

Here's one theory:
When your remote lock went out a couple years ago, did you ever check the fuses?

It could've been indicitive of the solenoid starting to go bad. If the solenoid got bad enough where it was seizing, your manual key turns could've dislodged the locking arm mechanism, hence the reason why there is no resistance anymore.

 

I did that same project a few months ago. In your lock mechanism there's a little horseshoe-looking piece with tabs on it. When you turn the key, the tabs grab hold of the door latch to open the door. Your tabs are broken off that piece.

Your best bet is to buy a used door handle (with the lock mechanism in it) on Ebay, take it apart while taking pictures of each step so you know how it goes back together, and replace that part in your car's door lock.

It's a couple hour job the first time around. Do some more searches on here and renntech. There are a few threads with details and even pictures.

Good luck!