This is the quest in Farangel's Landing, just outside Wayrest where you're trying to save a woman called Miranda Arcole. If you don't want to know more please stop reading here, otherwise I'd have to keep using spoiler tags which would make a mess of the post.

I got to the point where you enter the manor house and you need to kill the 'boss' but he beat me. I respawned at a wayshrine but then ran out of time to carry on and logged off. Later I went back to attempt the quest again and this time the boss wasn't there. I assumed someone else had killed him and dashed past where he appeared hoping to complete the quest before he respawned. When I got into the room where Miranda is she was lying dead on the floor and when I approached her I was told I was too late to save her and given the quest reward.

I looked up the quest out of curiosity and all the guides I've found say you save her. I don't mind that I wasn't able to save her (I actually really like that you can fail like this sometimes) but I'm curious about when it all went wrong.

Was it because the boss killed me first? Because I left the house? Logged off and came back?

If anyone knows it would be really helpful, especially if I encounter a similar quest again. (Or next time I do this one.)