Reference: I’laam al-Mu’aasireen bi-Fataawa Ibn ‘Uthaymeen – Page 186
Question: A person undertook an oath to slaughter a particular sheep, however, that particular sheep died without any negligence on his part. So is he [now] required to replace [the sheep and slaughter] it?
Response: If it died without any negligence or extenuating factor on his part, then nothing [further] is required of him.