We have sound but no light after LCD and digitiser fix?

Hey,

My recently replaced the lcd and digitiser on my friends iPhone 4. It worked perfectly afterwards.

She dropped it when she got home. And now its back with me to try and fix.

There is volume, nothing wrong with the phone itself, but all i have is a black screen.

I have plugged up iphone to computer and the phone is fine.

I have plugged in mains and we are getting volume from alert on phone when placed on charge.

Can anyone assist me with one of my more challenging fixes please?

I'm new to fixing phones and would appreciate some people to talk about it with me.

Kindest Regards