1 door of 3 panel slider no longer locks

Thanks for reaching out to us. Your photos show a mortise lock and handle, but I'm not sure which one you need to replace here. If the issue is that the door no longer locks, then either one of these could be the culprit. You're going to have to experiment to determine which one.

In the meantime, I can suggest replacements for both. Review the dimensions for our 82-226 mortise lock and 82-030 handle set to verify that they match the originals.