So me and my friend are having fun just hitting the random button and playing a random champs. but for some reason there is no random button for me anymore and I don't know why. We have won 5 in a row BTW, but can someone please tell me how to fix this.

If you click "Lock in" without selecting a champion, it automatically assigns you to a random character. Hope that does what you need.

It seems as though the random button has indeed disappeared. However, by locking in without selecting a champion, you are automatically assigned a random champ. It's a little harsher, but it still works if you just wanna be someone random.