The most likely guinea pigs to suffer from this are those housed in hutches at the bottom of the garden during a hot spell in summer that have no shade. Such hutches quickly turn into ovens with the temperatures soaring.
I have also seen a case and heard of others of people leaving their animals, as some do their dogs, in locked cars to cook in the summer time!.
'But they come from South America, don't they?' I have often heard said, when I tell people that guinea pigs are very prone to heat-stroke. They do indeed and it does probably gets even hotter out there than it does here. However, they are not imprisoned out in the open in a little wooden box!. Like most wild animals their senses are attuned to the weather far more acutely than our own. By the time the sun reaches it's zenith they will have found a cool spot to shelter from it.
The symptoms of heat-stroke are that the animals will be flat on it's belly, unable to move, breathing is shallow and the pulse is rapid and very weak.
Treatment must be immediate for the animal is very close to death. Soak a towel in cold water and wrap the guinea pig in it, or get a bucket of cold water and continuously sponge the animal down. If there is one to hand, an electric fan should be played on it at top power.
If you are not too late already, recovery can be amazingly quick. The guinea pig will shakily try to get onto it's feet and it should be assisted and supported under the belly once its up. At this juncture, the water treatment should cease for it could swing the other way and chill the animal.
As soon as it is able to, give it rehydration treatment, water if you have no rehydration fluid to hand. The best method of course is something like Ringers solution subcutaneously by hypodermic needle.
Those that pull through the ordeal usually make a full recovery with no bad after effects.