In Million Lords, seasons allows you to start fresh every month, leveling your hero and building your kingdom from scratch while keeping your previously earned items, chests and sapphires.
At the end of each season you will get rewards according to your rank. You will keep your items and sapphires but your level, your kingdom, the leaderboard and the map will be cleared.
Each new season give you another opportunity to improve your strategy and become a legend!
So if your account dropped to level 1 it's probably because a new season started.