I have good reasons to believe that this particular instrument failed during an FW update from v. 2.17 to v. 3.0.
(I was very tempted to buy this very instrument myself, but decided not to do so. The odds of fixing it seemed to bad)
The mains power failed shortly after initiating the FW update procedure.
The previous owner has informed that "I’ve been in touch with local representatives for Keysight and they say that the repair cost equals the cost of a new instrument. It’s unbelievable. If it comes to that we will consider another instrument vendor."
As the keypad is not working it seems hard to update the regular way via USB flash drive. I would check out if it is possible to connect to the instrument from a PC via the front USB.
I would try to find a way to establish a connection to the instrument via LAN. This may be hard because LAN was probably never set up before the miserable FW update attempt.
If you are lucky you can get access to the part of memory NAND containing the firmware recovery file. See enclosed picture.
Here is a link to the last working FW v. 2.17 on this instrument if you want to attempt to restore FW v. 2.17
https://www.keysight.com/main/software.jspx?cc=GB&lc=eng&nid=-11143.0.00&id=2367633&pageMode=PVI wish you the best and hope you succeed in fixing this awesome instrument.
Nixxon
EDIT: More info from previous owner:
" I have been updating the DMM ever since the first update and it has worked well. However, this time there was a power blackout right after update to V 3.0 was started. The DMM has two steps to get operational (as I see it). Step 1 is something like starting a BIOS and step 2 is loading the firmware. Now the DMM finishes step 1 and the then halts with the message shown earlier. This means I cannot access the front panel user interface. The DMM seems to be detected when connected to the USB interface but that doesn’t help much."