Camera problems in After Effects
It seems I was a complete noob ( for I am new with After Effects ). I couldn't see my composition because the blur on the camera was turned to 100% and everything was completely out of focus. Also, I couldn't zoom in on a specific layer because the camera was underneath them. This video helped me a lot to understand the way camera works in Adobe After Effects, and I wish I had found it earlier because I am dealing with this problem for a while now.
Thank you @JasonConrad for trying to help!