hi guys m really deep in this mess.
last week i got a firmware update to marshmallow, my phone's battery was about 70% so without connecting to charger i just started updating it, everything went well until the screen displayed "optimizing app 145 of 160" and it got dead. I thought the battery might be dead but i had some imp data that was not backed up. I kept looking for some way to boot my phone without loosing data but failed. So finally from boot menu i factory reset the phone and phone started with marshmallow like nothing ever happened except for the data loss. Later I read somewhere i could recover the data if I rooted it, so i Unlocked the bootloader but was not able to root it for some reason. when the rooting failed it went in boot loop it kept restarting, so i again custom booted with kitkat 4.4.4 (britain) and the phone started well. Now through settings/update/ i looked to update it to lollipop, while updating to lollipop i had my phone connected to the laptop but suddenly it went dead and now nothing is showing up not even boot menu. I have tried the following ways all failed.
power button and volume down for 2-3 mins/ and then connecting charger
leaving phone to entirely drain for a day and then charged it for 6 hrs
disconnecting and connecting battery
but nothing showed up except for 1 time when i connected charger of nexus 6 the led light glowed for few seconds.
Any kind of help will be greatly appreciated.