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|32840|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.