The computer suddenly shut down for some unknown reason.
Tips
grammar
Functions as an adverbial. Position: subject + 不知何故 + predicate, or sentence-initially: 不知何故，他... Tone is neutral — just flags an unexplained event. Colloquial equivalent: 不知为什么.
usage
Slightly formal or literary compared with 不知为什么. Fits written reports and narrative prose well. Don't use it for things you could easily find out — it implies the cause is genuinely unclear.
