Hi,
Here are the ifixit [[Topic:Nikon D3100]] guides.
Hopefully the motherboard repair guide will get you far enough into the camera to check that the CCD sensor connection is OK or to replace the [https://www.ebay.com/itm/Original-CCD-Image-Sensor-CMOS-Assembly-for-Nikon-D3100-Digital-Camera-Repair-/263876212443|CCD sensor- example only] if necessary.
The part link was only to give an idea of the cost of the part. There are other suppliers online that may suit you better. Just search for ''Nikon D3100 CCD sensor'' to get results for suppliers