I got this a few weeks ago. Here is what Riot said to me:
RiotTyphon (League of Legends Player Support | Riot Games Support)
Oct 17 19:24 (PDT)
It sounds like you experienced what we refer to as a ghost game. It is caused when the game server you were playing on fails to communicate the game-end message to PVP.Net.
The ghost game will last a maximum of 2 hours after this occurs. It often ends much earlier because the game server is simply backed up. Sometimes this issue can be fixed by relogging into League of Legends. Occasionally, restarting your computer may help. These actions force the client to request a new update.
Another side effect of the Ghost game is a delay with receiving your Influence Points, Experience or ELO. Once the game "clears" the server, these values will be updated on your account accordingly.
If the issue persists or occurs frequently it may indicate a problem with your connection. If this is the case, I would be more than happy to help you find the cause. If it was just this a one time occurrence, it was likely due to a delay in our server.
I am sorry for the inconvenience and wish you luck in your future matches.