There are a couple of reasons why a chat convo disappears from your list.
The other user has removed you from his/her list (chat will be transferred to archived list).
The other user has been banned from the app (chat will be transferred to archived list and a banned user indicator is visible).
The other user permanently deleted his/her account (chat will be gone and will leave no trace in the app).
Can you also tell us the name of your donor so we can confirm if his/her account was deleted. Otherwise, check your archived list to see if the user is there.