How did King Ethelred die?
I assume that you are asking about King Ethelred II of England, better known as Ethelred the Unready. He was king for most of the period 978 to 1016 AD. The exact manner of Ethelred's death is not known to us today.
Ethelred is most famous for having been ineffective in trying to protect England from the Danes. During Ethelred's rule, the Danes renewed their invasions of England. Ethelred tried to buy them off, but that did not work. He then provoked even more invasions by committing atrocities against Danish settlers. Ethelred was temporarily deposed as king in favor of the Danish King Sweyn. After Sweyn died, Ethelred was invited to rule again in 1014, but died in 1016. There is no record of how, exactly, he died. His son, Edward the Confessor, later ruled England until a few months before the Norman Conquest.