That's a bummer and it's pretty strange how a PLC would suddenly lose its memory?
From the picture I can see that is a Mitsubishi PLC , they are fairly reliable, and I have never heard of a PLC to suddenly have a memory wipe.

It's a real shame your elevator company didn't download the logic from the PLC and save it to a PC, then it would be a straight swap over. Those PLC arent too difficult to source...