It means that the game crashed for the player that attacked your base...in the middle of his or her attack. For that player, they lost their troops and the 21 medals. They would have been in the middle of an attack and the game just cut out and restarted. This problem has existed for more than 6 months when the game was in Beta mode in Australia and the Phillipines...and unfortunately Nexon have still not fixed it. One way to minimise it happening on one of your attacks, is to restart the game before you go on the multiplayer battle mode. The game seems to crash more often if you do a lot of battles in a row, or move on to new players to attack.