He was not a happy lad and did not like life in the palace where he grew up. When a trumped-up charge of attempting to rape his stepmother Phaedra reached his ears he fled in a chariot.
THESEUS prayed to the Gods to stop him, and POSEIDON, being nearest as the chariot was hurtling along a coastal road, sent a huge wave, which startled the horses into causing a crash with fatal results.
For a time, anyway. When ARTEMIS, his patron Goddess, heard about it she was livid. She demanded that her client be restored to life as he was innocent of any crime and POSEIDON had not been asked to destroy him anyway.
She raised such a commotion that ASCLEPIUS rushed to the accident spot with his first aid kit. A shot of Ambrosia had HIPPOLYTUS sitting up and twisting his broken neck until the pain went.
This in turn upset the FATES, who had not been consulted on this emergency raising of the dead. It meant taking in a lot of extra sewing and having to unpick lots of threads — and someone would have to pay. They threatened to go on strike unless someone was stitched up, so ZEUS had to kill ASCLEPIUS with a thunderbolt to keep them happy.
The FATES are still wondering about this, but the Ambrosia is never going to wear off and HIPPOLYTUS is now an ancient hippy sitting in the sun and enjoying the simple life forever.