noun
something on one's mind; concern; worry
Refers to a specific matter that occupies one's thoughts, often with a nuance of worry or preoccupation.
最近、仕事の考え事で夜も眠れない。
Lately I can't sleep at night because of work-related worries.
彼は
He seems to have something on his mind and has been silent the whole time.
noun
deep in thought; absorbed in thought
Describes the state of being lost in thought, often without a specific object of worry. Common in phrases like 考え事をする or 考え事にふける.
彼女は考え事をしながら歩いていて、電柱にぶつかった。
She was walking while deep in thought and bumped into a utility pole.
考え事にふけっていて、呼ばれたのに気づかなかった。
I was absorbed in thought and didn't notice when someone called me.