Guinea pigs do not cope well if temperatures exceed 30 degrees C. In particular if they are older, they find it more taxing. On hot days guinea pigs are best kept indoors.
If it has been a particularly hot day and your piggie has been left in warm weather and is not responsive and not moving, then immediately get a damp cloth and put your piggie on this while you rush straight to the vet. This is a medical emergency.
If you suspect heat stress as the cause, you need to bring down the core body temperature of your guinea pig in a way that does not cause shock. By placing your piggie on a cool surface this will start to happen, but it is important to move the position of your piggie regularly as the area against the cool cloth will become warm as heat transfers. Once with a vet they will administer subcutaneous fluids through a needle which can help. These fluids will keep circulation intact and more importantly major organs of the body. If your vet does not mention this – please ask about it.
One of the reasons that fluids are so important is that guinea pigs can’t perspire. In this way, they will overheat very quickly as they don’t have a way to cool.
Heat stress is a complete emergency. Many guinea pigs will not recover from this – so you do need expert help and quickly. If your guinea pig does survive, you need to ensure that you take adequate precautions so this does not happen again. For some time after they will be very sensitive to heat changes so you need to keep conditions very stable for them.