The Gryffindor vs Hufflepuff Quidditch game, to determine the Hogwarts Quidditch Cup, is cancelled abruptly by Professor McGonagall. She takes Ron Weasley and Harry Potter to the Hospital Wing where they find that Hermione Granger and Penelope Clearwater are the latest victims to be petrified. Percy Weasley is stunned that one of the Hogwarts Prefects could be have been attacked (CS14).
They have been petrified by the Basilisk, but no one knows this yet.
From the Web
Writing by J K Rowling on Pottermore: Chamber of Secrets
Pottermore feature: Chamber of Secrets collection
Harry Potter Wiki: Basilisk