Please take your Guinea pig to the vet tomorrow for a diagnosis. It could be anything from an injury, arthritis, paralysis or a stroke.
Have you been weighin her to make sure he isn't losing weight if he's not eatin much? A Guinea pig off its food is serious as they need a constant food intake to prevent their guts from shutting down. If she is losing weight and not eatin please step in with syringe feeding Asap.
Complete Syringe Feeding Guide
It sounds like she is also pining for a companion, which will make her life fairly miserable. If you aren't willing to get her a companion, have you thought about surrendering her to a rescue where she could be rehomed and have companionship again?
If you could add your location to your profile that would be great, we may be able to offer you more specific advice as to vets and rescue centres.