From the information Harry Potter and the Chamber of Secrets
, it is clear that Voldemort (Tom Marvolo Riddle) had opened the chamber and released its deadly monster, the basilisk. Instead of being petrified, Myrtle had looked into the monster's eyes first-hand, and was made into Riddle's first horcrux. There is a reason Riddle had attacked Myrtle.
What is her blood status?