Hi,
LED number 6 on the NCU is excellent news- that means the PLC/NCU system is powered up and functional. The problem is only with your hard disk.
Regarding LCD- do NOT frequently restart your machine to try to get it to work. Its a simple replacement. You need to open up your PCU module to see the LCD model number, and order a replacement via ebay or some other similar website. Repeated restarts risks your 840D system as the backup there can crash on frequent restarts.
You need a hard disk with the operating system and system programs available on it. Buying a blank disk will not help unless you have the backup available from someone else. I am trying to contact my local Siemens friends to see if anyone can help with this. If we can get our hands on this, I will send you a norton ghost backup which you can then load onto a new hard disk. Try out the machine using the external LCD, and repair the internal LCD later on whenever time permits and once your parts arrive. The normal problem in the internal LCD is that the backlight or backlight driver fails. I have replaced one machine backlight with a regular LED strip temporarily instead of using an external LCD- however, I do not recommend this for you.
Replacing the LCD with a new part (these are usually sharp/toshiba LCDs) does the job.
This person has the same MMC in stock with hard disk-
https://www.ebay.com/itm/122053249972 you can contact him and ask him if he can send you a clone for which you could negotiate some amount. If he's charging a lot for the backup, make an offer for the full unit which you can then keep as a spare.
For the hard disk, I suggest using the following
https://www.ebay.com/itm/203101179448 . The PCU will not be able to recognise the hard disk as it is- when you clone the hard disk, make sure you select the partition sizes being same as the original. I think your original hard disk is only 170mb, which means your partitions would be 40-50mb each. Please follow the same (the software will ask you what to do with the free space- do not allocate it to the partitions as the system would not work otherwise)
Best regards
Jaideep