No Operating System on my mobile

I had a rooted HTC Desire 816 with android lollipop and stock rom. To get stagefright update, i tried to replace twrp recovery with stock recovery. Now, just the powered by android boot screen is coming and nothing else. I cant shut down the phone. It doesnt go into fasboot mode either. i went to htc service centre. they are asking Rs. 9000/- or 6 for repair. Help

Auxus Aura A1 ( i am trying to change the operating system but i cant ... help me)

hello my dear friends,

i am having Auxus Aura A1 device which is holding Android 4.2 Kit Kat version i am trying to change this operating system can anyone please help me to change the operating system ... please...