Can not leaving the house Watch
Outside tends to be where we get what we need and do what we do so being inside all the time and separated from that can cause issues. For some people there isn't much option but to stay inside all the time so they have to adapt to get and do what they need without going out.
The main thing that comes to mind is not getting enough vitamin D. The main way we get vitamin D is from sunlight and obviously being inside all the time you tend not to see much of that. Being low in vitamin D can cause a few symptoms including tiredness. There's not really any other way to get the vitamin D we need (we get some from diet, but not as much) so unless you're taking supplements it's quite likely that staying inside for long periods would cause some loss of vitamin D.
Being inside would also tend to mean not getting much exercise- even just casual walk around the shops or to the bus stop is more exercise than you get wandering around the house- which could basically cause you to become terribly unfit which could cause aches and pains and a weak feeling body. Not being able to exercise much could also cause things like weight gain.
Another potential issue isn't so much a health one as a mental health one and that's being bored as sin. Most socialising happens outside as do fun activities and just changes of scenery that keep out minds ticking. Being stuck in the same place all the time without much to do can get super super boring and that can actually have a big drain on mental health. That combined with not having much fresh air, sunlight, activity etc can even cause things like depression.