Fleets can become stuck in battle on the world map as a result of a lost connection to the battle server; you will not be able to re-join battle but the fleet will still appear red on the world map, as if still in battle:
Fleets that are stuck on the world map are currently ejected from battle every 15 minutes; after this time has elapsed, you should be able to pilot your fleet back to your base. Do not attempt to re-join the battle, as this could result in the fleet being stuck for longer than 15 minutes.