HTC one M7 stuck on Fastboot / Bootloader screen

Hi All,
I have an HTC one M7 unlocked. Two days ago I tried to restart my phone and it is stuck on the Fast boot /bootloader screen. I tried to use the recovery and Reset Factory options, but these are bringing me back to Fastboot screen. Took my phone to a gadget shop and they said OS has gone. Any idea how to proceed? I am a newbie to this forum and any help would be much appreciated

***LOCKED***
M7_U PVT SHIP S-ON RH
HBOOT-1.57.000
OS-6.13.707.107
eMMC-BOOT 2048M
Jan 7 2015,14:50:53.0

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

BOOTLOADER
REBOOT
REBOOT BOOTLOADER
POWER DOWN