What happened to Medusa's head after he saved Andromeda?

Medusa didn't save Andromeda per se. It was Perseus who saved Andromeda. Perseus slew Medusa, cut off her head, and used her head to turn the monster "[the] Kraken" into stone after Hades released the creature from its ocean prison when Andromeda's mother, Cassiopeia, compared Andromeda's beauty to that of Athena's, offending her greatly.