Sick Cause of death?

Last night I came home from being out all day. I had left the house that morning around 7:30. When I left I gave Reina fresh grass and water. She was fine squeaking and running as usual. I work very early so Reina was used to being up at 5:30am with me squeaking for her hay everyday that I had work. But today when I got up after my alarm I was greeted with no squeak. I looked over in her cage and she was laying down on her side all stiff. The night before when I got home I got her fresh grass and water again and she barely looked at it. This was out of character for her (of course guinea pigs love to eat) so I pulled her out of her cage and laid her on my chest. She didnt run or squeak when I touched her so I knew something was wrong. I tried to hand feed her the grass but she just looked at it. After about 30 minutes I put her back in her cage and said i'd take her to the vet Monday because I knew it was closed on Sundays. Before I could she was gone. Im so sad I feel like I just lost my baby. Does anyone know what could have been her cause of death?