HTC One M9 0PJA100 no Android after reset

Hi everyone,

I am stuck with a rather expensive HTC paperweight and I hope to get some guidelines from a kind soul here.
I had this phone (functioning perfectly) for a while and recently I tried selling it to someone. Just one day later, the guy comes back to me asking for his money back and claiming that he "tried to do some updates and it failed".
Now the phone is stuck in a white background and colour text menu state. This is what I get on the screen :
*** Software status: Modified ***
*** UNLOCKED ***
*** S-ON ***

<VOL UP> to previous item
<VOL DOWN> to next item
<POWER> to select item

REBOOT
REBOOT TO BOOTLOADER
BOOT TO DOWNLOAD MODE
BOOT TO RECOVERY MODE
POWER DOWN


Now, if I choose REBOOT TO BOOTLOADER, it does a restart and gets back to the same menu
If I choose BOOT TO DOWNLOAD, it goes to a red-blue-yellow text on black background menu with this text:

hTC download mode

*** UNLOCKED ***
htc_himauhl PVT S-ON

LK-1.0.0.0000
RADIO-01.01_U11440061_52a.01.50225G_2
OpenDSP-v20.2.6-00452-M8994_0216
OS-1 .32.401.3
Feb 27 2015, 15:11:23(493992)

system info
show barcode
reboot to bootloader
reboot to download mode
reboot
power down
_____________________________________

Security checking failed NOT_FIND_SMART
IO
SD NOT MOUNTED

If I choose BOOT TO RECOVERY MODE I get to something called Team Win Recovery Project v2.8.7.0 and 8 main tiles , each leading to a sub-menu.

Given all the info so far, could you, please, point me towards a procedure/tutorial to get the phone back running. I am not/never was interested in tuning/upgrading the phone in any way other than stock. All I want is a normal, usable HTC M9.

Thanks in advance and I hope for a positive answer!