Hello, I am Dr. Zoe. This is very concerning. Is she giving you any signs other than not eating and preferring to not be active such as loose stool or difficulty breathing? I also would like to know what diet she is on (pellets, hay, vitamin C supplement, fresh foods).
The bottom line is that not eating is often a sign SECONDARY to whatever the primary problem may be. I am afraid I may not be able to guess based on simply being lethargic and not eating what her primary problem is (it could be anything from dental disease to bladder stones to kidney issues to cancer); this would really require a good exam or another more specific problem for me to guess. But the not eating is something you need to address right now.
With not-eating this causes some signifcant problems that are important to understand in order to know what it is so crucial we get her eating again. Guinea pigs are designed to be eating all the time. Their system is always running/digesting. So, when a pig stops eating the entire digestive tract comes to a halt. The bacteria naturally in the system overgrow and release gas (painful) and toxins, which cause a pig to get even sicker. Ideally, therapy for this involves a combination of hydration (best done by your vet with subcutaneous or under-the skin fluids), antibiotics, promotility drugs, pain relief drugs and you syringe feeding at home. Now, you can start some of this at home. But I do want to be very honest that if we do not get her going again, she really may not make it especially if she has been this way for longer than a day. This is what we consider an emergency for a guinea pig.
Start syringe feeding her at home- to do this you can either purchase the Oxbow Critical Care Recovery Herbivore diet at your vet or a pet store (not prescription). I like the Find Grind version since it is easier to syringe feed. This is a diet you just add water and then syringe into her mouth. If you cannot find this then use people baby food of the fruit or vegetable varieites. You want to also get a 6 cc (1ml = 1 cc) syringe to use for feeding her (can purchase at any pharmacy). You then want to syringe into her at least 12 to 24 ml (that is 2 to 4 syringefuls) of food every 6 to 8 hours until she is eating on her own. You can also syringe into her an electrolyte solution such as Pedialyte for children (fruity flavors are best). Remember guinea pigs cannot vomit so if she swallows it stays down. There really are not over-the-counter medications that will help this situation. This is about the only thing you can start now. I really would discuss if it is possible to get her seen by a vet.
You are welcome and I am so sorry to hear that she passed away.