I don't see the "silver" internal mortise latch you speak of. You said you did put a screw driver in the latch, but could not get it to unlock the door. Try turning the latch again and push the back of the sliding door toward the jamb were the keeper is installed. Maybe by taking off the pressure on the keeper you will be able to unlock the door. Another thought might be to go out side and try using a crow bar or flat bar up under the siding door near the lock to see if you can free up the lock. Take note when using a crow bar under the door use a small piece of wood under the bar so not to damage the track.
The silver box I referred to is in the picture I sent of the door and frame. It is barely visible because the door cannot be opened more than a crack and the box is visible in the small crack only.
I did try the latch turning while pushing the door toward the jamb before I wrote in. Will try to raise the door from outside and see if that helps. Thanks for the suggestions.
Was able to get the latch open by more vigorously and repetitively shoving the door toward the jamb while turning the latch toward the open position. Took the mechanism all out of the door and then to the local business which put the door in. They replaced the mechanism from the door, which I installed. Now all is good!
Thanks for your help.
DB
Great! Glad you got everything working.