Unfortunately, the mortality rate for many species of baby birds is extremely high, some more than 50%. It is sad, but definitely a normal part of nature. Even though there are only two babies left, this doesn't necessarily mean the mother can't or won't be returning, and doesn't necessarily mean anything is wrong. If it becomes more likely that the babies have been orphaned or abandoned, contact a wildlife rehabilitator. Raising orphaned birds is extremely difficult, and their chances of survival are extremely low, even with the best of care. Their best chances, outside the care of their mothers, is with wildlife experts.