When your district is damaged it may not be upgraded until you have repaired it to full health. You may repair your district by opening up build mode. This repair will cost you gold and the price may vary depending on how much gold per tile you are currently earning. However, If you do not have enough gold to repair your district you may use your gems to overcome the shortfall of currency in your kingdom.

 

While your kingdom was being attacked, your district was just destroyed and now needs some rebuilding. To do this open up build mode. From there you will have two options. First, wait for the timer to count down and your district will return to its original state or you can skip the wait by paying with gold.

 

While your kingdom was being attacked your district was just claimed. To get back what is rightfully yours, you must land on the claimed tile for that specific district. Once this is done the district will return to its original state. For example, if it was at level 2, it will return to level 2. If you would rather not wait, you can also pay gold to reclaim the district.