Rest in peace Kirby...

I'm more than devastated, words can't describe it. I don't understand why she died, yesterday night she was fine, she ate her dinner and came on my bed for pets as usual while I was watching a movie. She was her usual sweet, happy self... and this morning I find her dead in her litterbox. WHY? I've always done everything for her to be healthy and happy, and she wasn't even 2 years old yet... I don't know what to do with myself right now. She was the prettiest girl and I loved her SO SO much.