Every so often Chess.com needs to restart the servers for maintenance. When the servers are restarted, any live games (bullet, blitz, or rapid games, Daily games are NOT affected) that are being played will be declared aborted. These aborted games will not provide any rating change, and will not show in the game archive. They will not count as a win or loss, it will be as if they never happened.
To avoid this happening, server restarts are announced 40 minutes in advance by a banner at the top of the screen. If you try to start a game too close to a server restart, you will see this notification warning you:
Server restarts are not regularly scheduled, and happen on an as needed bases, but they usually happen about once per week, between the hours of 1AM and 3AM Pacific Time, and typically last only a few minutes.
This means, if you were to see the banner, you'd only have to make sure you weren't in a game at the listed time, then wait a few minutes, and everything would be back up and running!
What To Do
Server restarts help us keep the servers in top shape! If you see the announcement, simply avoid playing at the set time for the shutdown, and you have nothing to worry about!