There are several reasons that may be cause your pig to stop eating. One of them can be a change in their environment for example new house, new cage, new pellets or hay. There are several questions we need to ask ourselves before making an statement. Is the piggy a new piggy in the house? Is it a baby piggy? If you just adopted/bought a guinea pig and you brought him/her to the house he will be scared and he might not eat for a day or two it all depends on the cavy(s) he/she is scared and he might be getting familiarized with the environment, it also can be that he was taking away from his/her best buddies and the piggie feels sad or alone. I would recommend to house the new piggy after 2 weeks of quarantine with a SAME sex piggy. If the piggy is a baby, he might feel scared because he had mommy next all the time to eat together, drink water together. Although if you really notice your piggy has not eaten in 2-3 days take your piggy ASAP to a vet check up, he might be also sick and with forced food his sickness will not go away. Before any treatment also check the antibiotics the vet will prescribe, cavies are allergic to most of them and they can kill your piggy.