“She’s not dead, Argus,” he said softly.
Lockhart stopped abruptly in the middle of counting the number of murders he had prevented.
“Not dead?” choked Filch, looking through his fingers at Mrs. Norris. “But why’s she all — all stiff and frozen?”
“She has been Petrified,” said Dumbledore (“Ah! I thought so!” said Lockhart).

Harry Potter and the Chamber of Secrets - Chapter 9 : The Writing on the Wall
See this quote in the Pensive -