I was rewarded a gift, but it is nowhere to be found!

Hello! I logged onto League today after school. I went to go get a drink while it was logging on, and when I got back a minute later, there was a song playing I never heard before on League, and a chest with writing! I was so happy, and I am pretty sure it said Sad Robot Amumu, but I am not 100% positive. There was a message saying how I performed very well when faced by toxic players, and for that they rewarded me with a mystery box. I was so happy because I never received a gift before on League. I immediately went on Amumu, (and even bought him because I didn't own him previously), and surprise, all his skins weren't available. The one I thought I received, (the Robot one), was also locked. I was just wondering since it is a possibility that it isn't the robot skin, that maybe there was a way for me to look at all my owned skins, so I can see just what I got. Maybe it wasn't a skin at all! Nothing showed up in my recent purchases tab, so I was just curious. Is there something that I am missing? Like if I have to "open" it myself? If anyone has any thoughts or info, I would be greatly appreciative. Thank you all, and have a good day.