Q: when take photo with flash on iphone 6 photo coming dark.any solution please?

Hard to say what damage may have been done while you were taking your phone apart. There's a pretty good chance you've damaged your phone beyond repair. Apple will no longer service it, even on an out of warranty basis, something they might have done before you did the surgery.

How much light was in the room other than the flash?

The flash on an iPhone is not powerful enough to take any sort of decent picture in a completely dark room.

Turn the lights on in the room.

Saeed N wrote:

 

if i take this with out other lights is coming like this black

if i take with lights it coming ok

The LED (it's not a flash) in an iPhone only has a range of about 2 meters.