I am trying to hard reboot my phone (by pressing volume and power button together), but it is not working at all. No any option I got for reboot.
Previously it was working but now not working.
earlier it worked because the fastboot of kitkat provided that option.it's not available in lollipop fastboot.
to reset do it from settings>backup and reset>factory reset.
or you can do it from recovery mode by gettings in fastboot>navigate by vol down key and select recovery mode>on getting a dead android robot icon press and hold power button and vol up key for few second then release>now select factory reset from the menu appeared.
after reset first boot may take 10-15mins. be patient