During gameplay your computer maintains a constant connection to the League of Legend servers. If your computer doesn't hear from the servers for a specified amount of time(generally 15-30 seconds) it will consider the connection closed. Therefore, maintaining constant contact with the server is very important. Any outages or long delays in your communication with the server will cause a disconnect. While disconnection can sometimes be due to server instability, if it occurs often it generally implies a problem on your network.
Preventing Frequent Disconnects
Occasionally lose connection to chat or receive Session Closed errors
Get an in-game message that you have disconnected or timed out
Session Closed after "Skip Waiting for Stats" screen
Troubleshoot your connection #1 Solution
Disconnects have two major causes. The first is an unstable connection and the second is a very high latency("laggy") connection. The most common resolutions are moving away from a wireless connection and removing bandwidth intensive programs from your connections. Luckily we describe both these processes in our Improving Lag, Latency and Ping guide. If these fail to resolve your problem, please take the time to follow the other instructions in that guide. Tweaking your connection for maximum performance is the #1 way to resolve disconnections.
Restart your modem and router
Like your computer, modems and routers can get sluggish after long periods of extended use. Restarting them can flush their caches/memory and help them perform to full capacity.