This is one of those questions that is very difficult to answer via an email/online - the initial questions I would ask would be along the lines of:
Is it showing any signs of ill-health ? - check your guinea pig all over for signs of injuries or illness - if you suspect an illness take it to a vet asap for a thorough check-up. A change in behaviour - eg becoming quiet is often one of the first signs of ill-health - has it been outside and eaten something poisonous ? has it fallen off something ? There are many variables, think of everything you can.
Has it recently lost it's piggy friend ? - if so, it could be pining/missing her. Is there any possibility of you getting another pig to keep her company? Make sure you give her lots of attention, exercise, and keep her entertained so she isn't bored.
Is it a new guinea pig ? - if so it could be quiet and nervous and it will take time and patience to help her settle in.
Is it very old ? - Is it housed outdoors in the cold of winter subject to wind and dampness ? - is it being fed/cared for properly ? etc.
As you can see, there is no definate answer to this question - only you can do the determining and investigating really, with the inclusion of your vet if you feel your pig may be ill. The best advice I can give [as with most health queries] is that if your pig is uncharacteristicly quiet and you can't think of an obvious reason then have a veterinary check-up - SOONER rather than later.