Unanswered means having received no reply. An unanswered letter, questions left unanswered, unanswered calls.
The commonest use is about questions rather than letters: unanswered questions is the standard phrase in journalism for what an inquiry has not settled. Unanswered prayer is the older use, and it belongs to religious writing.
1adjectiveunansweredHaving received no reply.
“My letters to the council have gone unanswered for months.”
unanswered is a state.
Un- plus answered, from Old English andswaru, a reply — literally a swearing against, from a legal oath.
Middle English
unansuered
English, today
unanswered
From Middle English unansuered, unanswerd, equivalent to un- + answered.
Etymology from Wiktionary, CC BY-SA