Was his cage not heated then? I'm assuming not as you've said that your apartment was cold- which if the cage was heated shouldn't have affected him. The cage should always be heated at a near constant temperature. I use a CHE set up which didn't cost very much at all and keep his cage between 22 - 24C. He could very well have attempted hibernation, they don't often survive this.
If the poop was soft with a tinge of green, this is usually caused by food or stress- if it was sticky or slimey then that could have indicated an infection in his gastric tract. But I've not really heard much on green stool being a major problem unless it's reoccurring.
In terms of cleaning, if urine is left and not cleaned over days, the material it soaks into becomes a bacteria breeding ground and can expose your hedgehog to ammonia. I know you said you did a spot clean, but I'd imagine a more extensive clean would need to take place to clear any bacteria potentially in the cage.
Otherwise, I honestly couldn't say what possibly caused your hedgehog to sadly pass away. I'm very sorry to hear this. Whether or not it was something that you could have prevented, I hope in future should you ever own another hedgehog, you can take some additional steps to avoid the chances of it happening again.
If the poop was soft with a tinge of green, this is usually caused by food or stress- if it was sticky or slimey then that could have indicated an infection in his gastric tract. But I've not really heard much on green stool being a major problem unless it's reoccurring.
In terms of cleaning, if urine is left and not cleaned over days, the material it soaks into becomes a bacteria breeding ground and can expose your hedgehog to ammonia. I know you said you did a spot clean, but I'd imagine a more extensive clean would need to take place to clear any bacteria potentially in the cage.
Otherwise, I honestly couldn't say what possibly caused your hedgehog to sadly pass away. I'm very sorry to hear this. Whether or not it was something that you could have prevented, I hope in future should you ever own another hedgehog, you can take some additional steps to avoid the chances of it happening again.
