I managed to reload the PLC from EPROM (switch the sw1 on the 805 to position 2, and press resart) Afterward the PLC stop and Missing data block message were gone.

Then after connecting S5 via port x122 with a 6FC9 344-4T cable I was able to compare the program from our archive. (Go to the data trasfer connection screen, select the "plc device" with 9600 baud, 7 data bit, 2 stop bit, rts/cts controlling, press read in start. then go online with S5)

I verified the FB 240,241,242,243,250, and 251 and moved them to the PLC as those were the only ones missing/not identical.

might have to load machine parameters but everything else is back to normal.

Your post really encourages me to resurrect my Sinumerik 805 controller on Padovani Lathe as my machine is not equipped with the Siemens Operator Keyboard. Step5 and right cable should do the trick 

Any idea where can I procure both?

I am based in India