I literally had this happen to me recently in ranked.
Our mid laner disconnected without warning about a minute into the match (before minions spawned) and never returned. I was supporting and doing just fine in bot lane with my ADC. We were even stomping the enemy ADC and support Ryze (I know...) pretty well and the other lanes were doing fine also.
However, it wasn't good enough for our ADC. He kept threatening to leave because of the 4v5, thinking it's a waste of time, never mind that he was pretty fed and the other ADC was shut down. Then finally, he says bye and leaves. If that wasn't bad enough, yet another ally decided to leave too, making it a 2v5.
I believe we would have won if he stayed. I was so upset about it since if I remember right, it was a series match.
Only my team, though. Enemy team never has an afk. :|