My Tortoise has died

Hi, i have 2 horsefield tortoises ages aprox 4yrs, this a.m i found one of them had died!

I am unsure as to how this has happened as everything was fine yesterday. She was quite a small tortoise yet when i found her this a.m, the only thing i can say to describe it, was as though something heavy had been dropped on her! She looked as though her shell had split into two layers too! Also there was some some not so pleasant stuff leaking from the damaged shell!
I have been tortoise sitting for my moms 3 horsefield, ( who can be quite angry at times) but i have been assured that another tortoise could not have done this!
Hope someone can shed some light on this for me!

Thanks