This can happen when you lose connectionto the game servers mid-battle. Your battle had started, but before you had deployed all troops your connection was lost.
In order to avoid that the game reloads at every small hiccup there is a short buffer period. This means when the connection is lost, your device has several seconds time to reconnect to our servers during which you can still command actions. If your device reconnects, all is good. If it doesn’t, the game will prompt you to reload.
Due to this delay, the outcome you see on your screen and the outcome you see after reloading the game (click the "return" button) can be completely different.
The game servers will only have registered the troops you deployed before losing connection and will play the battle to an end without your lead. Depending on the difficulty of your opponent this probably leads to a defeat.
Most, if not all, Commanders have experienced this at least once, and we know how frustrating it can be.
Before starting an important attack, hook up with a solid wifi network, or at least a solid 3G/4G connection with a reliable carrier. Full wifi bars or coverage alone is not always enoughto guarantee a solid connection, because local networks can slow down and get overburdened at times. This is especially true if you play in a moving vehicle or on underground public transport.