Lost kitty

I can barely write this. The last time we saw Indigo was Friday, May 23. He has been gone 5 days. He never goes anywhere. He always shows up in the morning for his wet food. He has access to food in the basement any time he wants. He would not stay inside the house but he was inside a lot.
Indigo is a happy, sweet cat. We don't know what has happened to him. I am not dealing with it well. My heart is broken.