Thank you, Jimmyd22.
Here's an update.

I went to the first mechanic to pick up the car.
they tried two different scanner to read my computer and failed.
they also tried another computer from a different car and hooked up to my car to see if that's the problem.
They claimed that everything's "fine" with another computer.
"Fine" means there's no code detected... (well i'm not surprised!!)
the mechanic suggested to drive some more and see if the same problem comes up again.
I agreed and came home.

Then, I used my OBDII.
It sure did show "unable to connect", even though I saw P0420 myself two days ago.
Then, I tried again...
Somehow it was connected/tried to retrieve the code and screen says no DTC stored.

well... it seems like the computer is reset.
i plan to drive some more and see if the same problem comes up.
man... too many possibilities.

i'll keep you posted.