If you don't have any quests available that can mean two things. Either you have completed all available quests for your current level (if so, levelling up will provide you with more quests) or you have completed every quest currently available in the game (this can happen on higher levels). If you do not receive new quests after levelling up, you simply need to wait for the next update as we are constantly working on creating new quests and additional in-game content.