。。
Stay quiet. Otherwise, your life is forfeit.
Literal
Be-silent-stay-[command-てろ]. Otherwise, life [topic-は] does-not-exist [emphatic-ぞ].
黙ってろ is the contracted/colloquial imperative of 黙っている — 'stay silent' rather than just 'be silent.' The full form would be 黙っていろ; colloquial speech drops the い. The follow-up さもないと命はないぞ ('otherwise your life will not exist') is a stock threat — 命がない ('to have no life') as a near-fixed expression for 'be killed.' Final ぞ keeps the rough, masculine tone.