iPhone 4S camera stopped working after screen replacement

Hi,

today I replaced the front of an iPhone 4S.

Everything works fine except the back (main) camera!

The shutter won't open. I tried a full reset (home+sleep buttons) but it didn't fix it.

I removed it and put the camera in again a several times but it doesn't work :(

What could I do else? Has someone experiences with that issue? Maybe a grounding issue?

Please help.

Thanks and best regards,

Tolga