prolonged silence
Refers to a long period without communication, especially when someone has not been in touch. Often used in the phrase 疎音を打つ (to break a long silence by contacting someone).
I haven't heard from him for a long time, and I'm worried.
I broke a long silence and contacted my friend after a long time.
Compound of 疎 (so, 'distant, sparse') and 音 (in, 'sound, news'). The exact historical derivation is uncertain, but the spelling is conventionally associated with the meaning of infrequent communication.