Hi
As a repair guy (for over 8 years) I can set your mind at ease. I have done countless iPhone 6 repairs and simple connecting the battery to the motherboard can not damage anything. Doesn’t matter what’s connected or not.
The only way you could short something is when you connect the battery, turn it on by pressing the power button or connecting it to a charger cable, and then, when it’s power on, connecting the camera or the screen when the device is ‘hot’. The iPhone 6 motherboard is strong enough that connecting the screen, even when the device is on, wouldn’t cause damage if the screen isn’t lit. Iff it is, the backlight coils probably would burn, but as I said, never happend to me. And for all your DIY’s, DONT TRY THIS AT HOME, turn of the device and disconnect the battery before making repairs !! Just to make sure, as the 6S for example will surely burn out the backlight if you forget this
So, as mentioned in the comments, connect it to iTunes, to make sure it’s not the screen that’s stays blank (wait 30 minutes to make sure the battery wasn’t depleted).
Iff iTunes does notice the iphone check the backlight. hard reset and use a bright light reflecting in the screen, the apple logo is visible (but only barely). Iff so check the backlight soldering on the back of the LCD screen and its flex cable. If it looks fine you probably damaged the motherboard backlight coils.
Test with a new battery, with enough juice in it (all new ones have at least 40% charge). If you have a volt meter, test the current battery. It should score about 3,85 V or more, otherwise the iPhone hasn’t enough to power up.
Iff you used a housing with a new dock connector flex cable as well, maybe this is faulty, not powering the motherboard and the battery, so connect your old one and see what happens.
Iff you did all this and it still not working, than it’s time to buy a new phone.
Good luck
3 
Is it completely no apple logo?
Try plugging it into a computer. A couple things to try unplugging are the home button extension cable, power button cable and volume button cable.
Ben さんによる
My guess is the battery went dead or you shorted the logic board by mistake. Try getting a new battery and connect it to iTunes. If it doesnt show up i think its board level issue, and be careful to know where every screw goes. Common way to short the board is when you are connecting antenna cables.
Jesse Julian さんによる