When there’s an outage on League of Legends you’ll be one of the first to know on this page, as players will flood the internet to find out the problems, which would have occurred when playing the game on either PC or Mac. Usually it has something to do with the server going down, although there’s a few gamers that have mentioned login in issues in the past.

League of Legends Reports
@RiotSupport Hi Riot, is the LOL EUW down? the games don't launch
@RiotSupport are the servers down? Been afk on client last 20 mins and now it's trying to put me into a game that's already finished over an hour ago lol
@RiotSupport [LoL] Server down? "An unexpected error has occurred while attempting to join the queue". Sign out and restart not fixing the issue...