Screen turns on only for few seconds. Even replacement Screen

A friend has an iPhone 6 plus, and she sat on it. The Screen was not broken, but the case was bent, So i opened it, removed the screen, and gently bend the case to its proper position. Put back the screen, and.....nothing appears on the screen, although the phone starts.

I said im no Pro, let it have checked with a local store. They said its the screen, so we bought a new screen,

Installed, and the screen shows an image for a few seconds and turns black. I still not able to figure out under what circumstances the screen turns on. I disconnected it, connected it again, and it came up for a second.

The System itself starts fine, because i could connect with itunes and reset the device.

So did something else break? At the end maybe it was not even the Screen, maybe a Cable loose/broken ?

Besides: Tho home button cable is broken. But that was broken before, and she doesn't want to have it fixed right now., so i doubt it has an effect on the Issue here.

What options do i have ? The next Apple Store is 150 km away. Any cable that could be broken ?

- the case has been bent in upper left corner

- the local Store where we had it checked is not an apple store and i doubt he has profound knowledge. But there are no "real" experts closeby. (Costa Rica)