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が止まらないの

won't stop

→ が
phrase Rare #30,807
won't stop が marks what won’t stop. The sentence-ending の adds an explanatory or conversational tone.
JA: 涙が止まらないの。
EN: My tears won’t stop.

くしゃみが止まらないの。

I can't stop sneezing.

血が止まらないの。

The bleeding won't stop.

朝から咳が止まらないの。

I haven't been able to stop coughing since this morning.