make sure the guinea pig is getting enough food and water. it could have problems eating if the teeth are too long and need to be ground down at the vet. also, if nothing is stimulating the pig, i'd say to go to a vet asap so you can catch something early if it is sick. it depends on how long the pig has been lethargic, too. if it's only been a day or two, wait a couple more. the only 'downside' is that guinea pigs are considered 'exotics' and can cost a lot for vet visits. i still think it would be worth it. try calling the vet to describe symptoms first to see if they can offer some help over the phone. Buy a small book on Guinea Pigs because they do hibernate.