There could be a number of reasons why your cardinals have disappeared from your yard, but the most likely explanation is that they have migrated to a different area in search of food and shelter. Some bird species, including cardinals, will travel long distances in order to find the right environment for them. Another possibility is that a predator may have eaten them or that they died from exposure to harsh weather conditions. It's important to remember that wild animals are subject to natural selection and death, so it's not unusual for them to disappear suddenly from an area.