Why isn't my camera working?

Before anything else, you should restart your phone to see if that fixes it. Sometimes all your phone needs is a good restart. If it doesn't you have you will have to open up your device.

Having something stuck inside the lens seems unlikely since the phone is sealed, but it is possible. You want to open up your device following this guide. Once you get to the camera, check to see if there is anything stuck in the case. If there is none, get the camera itself for damage and scratches. If you notice a scratch on the lens, you'll need to replace it.

If there is any particles or dirt that made it's way inside the phone near the camera it can cause it to work improper or give a shadow on pics taken.

I recommend that you disassemble the phone carefully and use a can of compress air that you can pick up at walmart or online. Simple spray the compressed air and it will safely remove the particles that were bothering your camera.

Important that you do not touch the camera circuit it can cause ESD and result in short circuiting the board.

Use ESD strap or grounding equipment when working on.