Some carnivores are not equipped with a digestive system that will be able to digest plants. This means that they can eat to satisfy their hunger, but their bodies would not be able to get the nutrients that it needs so that they will be able to survive. Some carnivores come with really short digestive tracts, which means that digesting plant material will not be possible at all.
The short intestines will just allow the plant material to go through without getting the needed nutrients, which may cause starvation and death. There are some carnivores that would be able to survive without meat, though. It would depend on their digestive systems and what they are used to.
They will die! If carnivores eat only plants for a long period of time, they will die. As you may know, carnivore animals are animals that consume meat as a source of energy. Although some carnivores such as mesocarnivores (diet consist of 50 to 70% meat and balanced with non-animal) and hypocarnivores (diet consist of less than 30% meat) can survive without meat for some time, obligate carnivores such as the wild cats (lions and tigers) cannot survive without meat.
Carnivores have a specialized digestive system that is short and unable to process plant-based foods. If they consume plants, these animals are probably going to excrete the waste. Over time, these animals become deficient in essential nutrients; this can lead to diseases and ultimately, death.