。
My nosebleed won't stop.
Literal
Nosebleed [subject-が] not-stopping [polite-ません].
鼻血が止まりません — 鼻血 means 'nosebleed.' 止まる is the intransitive 'to stop.' The polite negative expresses an ongoing medical concern.
。
My nosebleed won't stop.
My nosebleed won't stop.