My little bird died, I can't stop crying?

We first found him 2 days ago, it was a small baby bird. We tried to find its mother but couldn't, so we took him home. He had most of his feathers and was learning how to eat on his own and flying a little. When I feed ed him, he opened his mouth and it was very cute. I held him in my fingers, and sometimes he would jump off. He chirped a lot. We bought him a pet cage and put him inside. I fed him 3 times a day. We put him outside so he can get some sunlight. But today we were going to feed him again, and we found the pet box on the floor, and the baby bird next to it. I'm devastated....I loved this little bird to much I feel I did something wrong. I'm so sad :(