I need some serious advice

I woke up this morning to find one of my rabbits had passed away. I'm devastated. While placing him in a box I noticed a small clump of poop stuck to his bum hairs. Was this GI stasis? I feed my rabbits a diet of mainly oxbow Timothy hay, a small amount of oxbow pellets and greens grown in my garden or organic from the store.
I need to prevent this from happening again, I feel like the worst person in the world.
His mate is now alone. I am scared she will become depressed. Should I attempt to bond her with another rabbit right away? She is spayed.

Anybody?

let her have time to adjust.

We can't tell you why your rabbit died.

You need to tell us what you observed about your rabbit in the days prior to it's death.

1. was it pooping and peeing regularly.
2. was it eating and drinking as per normal
3. could it have been frightened by something and died of a heart attack
4. was your bunny in good shape...not too fat? Fat rabbits can't keep themselves clean very well.
5. did your bunny grind it's teeth in the last couple of days? Appear bloated?

do know that you can't prevent all deaths.

I'm not sure if he was pooping as he shared a cage with his friend. He was eating fine but last night didn't come running up but I thought nothing of it. He didn't look bloated and he wasn't grinding his teeth. He let me pet him and that was about it. Nothing super unusual.

He wasn't fat, and only 5 months old. A lionhead.

then you just can't really know. Sometimes rabbits die for seemingly no reason.
If you saw no signs of health issues, then it could have been as simple as a noise happened, he got scared and then died.

I am sorry for your loss.

If he wasn't showing any outward signs of illness it's impossible to tell what killed him unless you take his body for a necropsy. Bunnies aren't built to be hardy and can deteriorate so quickly, don't blame yourself. It sounds like you were doing everything right diet-wise.

Regarding your female bunny, I have to disagree with ladysown. Don't let her stay in their old digs alone, she will miss him, wonder where he is and fall into depression. It's the same logic as people wanting to move out of a house after a loved one has died there. Take everything out of the cage and clean and sanitise it, whatever has his smell on it. If possible move the cage to a different location, but if not, rearrange everything so it looks and smells like a new home. Then try to take her on bunny dates within the week.

I let her hang out with my English lop. He's not neutered yet and just wanted to hump her. I allowed it for a short while and then separated them again. Thanks for your help guys. We do have dogs that occasionally bark so maybe he had a heart attack?

You just never know. We've had a few young rabbits, 3-4 months old, die rather quickly. The only indication there was something wrong was that they weren't acting their usual self and stopped eating. Through having one necropsied, we found that one had enteritis. That doesn't mean that's what happened to your rabbit, but it's a possiblity. Stress can play a big part in a young growing rabbit's health, but the possibilities of why yours died are endless.

okay thanks guys. I feel a bit better that this couldnt be prevented.
we buried him in the yard.

Sorry for your loss.