iPhone not working after replacing digitizer

Hi,

Hope you can help..

My friend had a iPhone 3G that was working, but had a cracked digitizer and no power button, but it did still work.

He asked me to replace the digitizer and the power button, so while I was at it I thought I would replace the back to the phone too. I also replaced the battery.

However, I have replaced everything and put it all back together and now the display doesn't work (which it did before)..

It does make a noise when connected to iTunes though!

Any ideas?

Thank you

Leo