More than likely the latch is stuck. Try some Liquid Wrench Super Lubricant, greenish/aqua can. (It has a lot of Graphite in it)
For now just use whatever oil you have handy and spray it on the latch on the door side and manually latch and open it a few times to work it in.
It's most likely inside the door where the problem is though. The outside door handle and the rod gives troubles quite often. But oiling the latch does help some.
Hammer and a screwdriver maybe?
That's odd, must be something jammed inside or it's that froze?
Or it could be inside the latch itself, in which case the door panel has to come off.
I'd try a little force with it first though.
My driver door latch failed a few months ago.....it would latch, but not lock either with the remote or the key. I sprayed the hell out of it with everything....nothing. The problem seems to be the spray won't penetrate into the part that's actually sticking.
It's possible if you took the thing off the car, it could be partially taken apart enough to lube the inside and free up what's stuck.
Mine got replaced under the basic warranty. $80-$120 if you have to buy one plus labor if you can't do it yourself.
Thank you for all your ideas. It turns out that my wife had driven through a lot of rain and then overnight the the temperature dropped drastically. The latch was literally frozen when it warmed up later in the day it started to work. You can be sure that we will keep the latches lubed.