Your troops will abandon your city if your food production has gone down to zero. To rectify your food production, build more farms in the 'field' section or level up the fields you have. For immediate food, find a barb camp (under the 'map' tab) and attack it. For a level 3 barb camp, you can send 5K cavalry with no loss of troops, to attack, you will receive 300K food upon their return. I'm unsure of the other levels/troop requirements. You can also always find unallianced players and scout/attack them for their food. Or if you are in an alliance, find out who your enemies are and scout/attack them for their food.