Why did my dog pass away?

Hello,

I am so sad that my dog passed away while sitter was my dog on the day we requested to watch her. She is only 5 years old and recent comprehensive test result said everything was normal 2 weeks ago. The sitter took her to the vet and we drove back to the vet as soon as we got to our getaway place, which is about 4 hours of driving from our home. I know nothing can bring my lovely fur back to my life, but the saddest thing is why she died and everything looked normal when we saw her at the vet.. I am just so sad and I do not know what to do.. I would like to find out what took her life...........