The same happened to me a few minutes ago, here's what I believe happened:
I started a URF game normally, black screen, no loading screen, nothing. After a minute I get an error "Unable to connect to the server. Please check your network connection and attempt to reconnect to the game."
The first thing that was weird to me about this is, I had an internet connection. Even after successfully logging in to the client (which requires an internet connection btw), I still got that message.
A few minutes into trying to reconnect, I suddenly see my lobby screen and I thought that the match must have ended.
Not only that but later I was looking at my opgg and I couldn't find that match.
Tl;dr I believe that the server-side game crashed and was terminated. A dev should probably look into that.
