This occurs when you were unable to join a game server within three minutes of your queue popping. You will not have your rank/statistics negatively affected when your server slot expires before you were able to join your assigned game.
Support staff are unable to clear your server slot lock for you and your only option is to wait until the game you were meant to join has been completed or 45 minutes has passed, whichever occurs first. At the end of the day, you are negatively affecting the players in the game by not joining in a timely manner and it would not be a very effective way of preventing those who want to intentionally ruin games if our staff unlocked your lobby on request.
In the very rare instance that your server slot says it has expired, when in fact, you have already completed your game or the game did not start, wait 10 minutes and then press the "Leave Lobby" button. You do not receive any penalty for leaving the game lobby and may freely do so once the server unassigns from the match. This may occur in situations where, for example, there was a CS:GO update so the game will not have been able to start as any new player will have a different version of the game than the server.