Can you have a necropsy done to her to find out? Dogs can stroke, have heart attacks, aneurysms, and other fatal conditions; just like humans. I'm very very sorry for your loss, and I do hope you find out the cause.
A stroke or heart attack could have caused sudden death. I'm sure there are other possibilities as well, maybe you want to ask your vet. I am so sorry for your loss. God Bless - Blu
So sorry to read this. Unless you have the Vet do a necropsy, there is no way to know fir sure unless she had some diagnosed condition. Just like us, animals that are older can pass fir a number of reasons. My best would be heart or stroke.
Hugs