Flat affect
Good news and bad news get the same tone. Their face barely moves.
Rule out first
Medication (antidepressants, antipsychotics) that blunt affect. Fatigue or illness. A cultural or family style of reserve. A neurological condition.
What could explain it
Expressive suppression: They feel it but have learned not to show it; in private or when relaxed, more comes through.
Major depression: New compared with their usual self, with loss of interest, poor sleep, low energy and hopelessness.
Emotional numbing: Follows trauma; they say they feel nothing about things that should matter and sometimes miss feeling.
Schizoid personality: Lifelong, with little interest in relationships, and they don't seem bothered by the absence.
Autism in adults: Expression doesn't match an inner life that may be rich; ask in words and they can tell you.
A next move
Compare with how they were a year ago and ask, privately, how they're feeling in general. A change from before plus hopelessness means depression; lifelong with contentment means trait; feeling nothing after a trauma means numbing.
If it’s you
Do I feel things and hide them, or have I stopped feeling them?