How to download the Photos app again?

I have problem with my iCloud and my images there, so I tried all kinds of things to try to fix it, one thing I did (witch maybe was stupid) was to delete the Photos app (because I thought if I download it again it might work again), but I con't find it in the App Store. Why? What do I do now?

I'm not so good at technology, the images in iCloud stoped working after I had to to a software update. Why is that? What shall I do?

*basically crying*