Complex sentence: Once Gerda reaches the Snow Queen's castle she has to solve a riddle by the guards before she can enter, and once Kay is healed by her warm tears, they solve the Snow Queen's word puzzle and are free to return home.
Split into simpler sentences: Once Gerda reaches the Snow Queen's castle she has to solve a riddle by the guards before she can enter. Then after Kay is healed by her warm tears, they solve the Snow Queen's word puzzle and are free to return home.