Be kind to yourself and give yourself time to go through the grieving process. Strong feelings of failure, guilt and a hefty amount of soul-searching are part and parcel of the onset of the grieving process. It is usually worse after a sudden death.
I don't know whether that is helping you, but if she wasn't coming out of the hut in the morning and was dead by late afternoon, her body had most likely already started to close down and there would have been nothing you or your vet could have done to stop this.
Unfortunately, I have been too often in this position not to have learned this the hard way. Guinea pigs have a much faster metabolism, and that is never as noticeable than when they are dying - that is also very much on fast forward and can happen suddenly at any age if some organ gives way in their body. All you need it a genetic time bomb ticking quietly away.
Try to take consolation that she could pass away peacefully in her familiar surroundings and that she didn't have to be rushed to the vets for a a messy and futile attempt to keep her alive against the odds or for an emergency pts/euthanasia, which would have left you equally shaken.
One of the life lessons I have learned over the years is to always make sure that all my piggies are coming to me to pick up a piece of veg in the mornings and evenings as a quick check. I still won't catch any sudden problems that happen within minutes and I still won't necessarily catch any more subtle issues if a piggy keeps begging and running away with their morsel until the moment they are ready to die; but it is a pretty useful tool and one you may want to apply as well in the future as a constructive lesson you take away. It is one of these things that you can only learn the hard way by bumping into it, so please don't start a new round of beating yourself up. We all learn as we go along; shaped by our own experiences, the good as well as the painful ones. I have found that it helps me to ultimately get past the guilt loop better if I can take a new 'life lesson' away from a rather unfortunate death and apply it in the future as a legacy to a particular piggy of mine and to hopefully save more piggies' lives that can still be saved.
I like to envisage my past piggies as my little furry guardian angels who inspire me to learn more and get better at looking after my current and future piggies as they are still present in my heart and in my memories and thoughts anyway and shape very much what I am doing now. They can't keep me from making new mistakes or misjudgments but they can help me to evolve as an owner. 'Experience' is very much the result of bumping into things the hard way, learning to bear the pain of the scars and your best to not bump into exactly the same bruise again. Experience is also a never ending life-long process. You don't come by it by sailing nice and smoothly along and by never putting a foot wrong... But I like to think that I have been able to help many more people better over the years than I would have been able to without those painful lessons.
HUGS
