First off, ensure your child safety lockout has not been accidentally activated. The lever will either go up and down or rotate left and right. However since you have stated that it is the driver's door, that most likely won't be your issue, as the child locks are only on rear doors where children would sit. But when I was a young diy'r and a budding mechanic, and I had a problem like this, I would pull off a door panel of a door that was working correctly and closely observe the differences of why the one was working and the other not. Without a video of your issue, the best I can tell you is that your problem is most definitely something inside that door! Concentrate on the linkages and plastic retainer pieces (that get brittle and break) and ensure that the rod(s) move at the interior door handle and that it moves at the latching mechanism.

 

"what else could the problem be?"

If you have confirmed the inside handle and the linkage are all intact, but stroking them doesn't pull the release lever on the door lock itself, then it is most likely that the lock assembly has an internal failure. You can get a used Fr LH Door lock from a scrap yard, or a pick and pull to replace it with.

you didn't mention if yours is power or manual
To help you with your search, the part number for a 2000 Corolla Fr LH Door Lock is:
69040- 02040 (manual)
69040-02050 (w/alarm)
69040-02060 (power)

There is a small snap where the Inside Handle rod attaches to the Lock, if that is broken, tell us what color the broken snap is and we can tell you what the Toyota part number for it is.
Norm

 

Open the door panel and examine the operating rods for the handles. I bet one has come loose or got bent. While you're in there lubricate the pivots with litium grease. Also lube the window regulator to retard it's wearing out.

 

OP said the trim cover is already removed and all rods looked fine.

 

Something has to be disconnected or broken.