What should i do about my Kittens going missing?

I have a Cat and 3, 8 week old Kittens. They have always hung about my garden, shed and in the house. However this morning when i got up, the 3 Kittens had mysteriously disappeared. The mother has been calling for them all morning and i do not know what has happened to them or what to do about it. Any advise greatly appreciated. Many thanks.