Why aren't my guide and other dead NPCs respawning in Terraria?

Solution 1:

There's three reasons why an NPC might not respawn:

1. They don't have appropriate housing.
2. Their spawn conditions aren't met anymore.
3. There's a goblin invasion going on.

Did a clown blow up parts of your base and make it uninhabitable to NPCs?

If not, there might be an invasion going on that you didn't notice, if you built your base away from the original spawn point (which is the very middle of the map). Go there, and start killing goblins until it says that they've been defeated.

Also, certain NPCs need their initial spawn conditions to respawn. For example, to get your Demolitionist back, you'll need to carry an explosive around with you until he comes back.

Solution 2:

I had in fact missed an obvious requirement for my house. The corruption has crept a tiny bit too close to my house for it to become habitable.

I simply pushed back the corrupted biome by using Holy Water(crafted by using 5 Pixie Dust, 1 bottle of water and 1 hallowed seed) which convert anything, even the corruption into hallowed land. Then to prevent the hallow from spreading too wide which would spawn monsters( I don't want monsters spawning so close to my house. Don't want a unicorn to run into my house and impale my NPCs), I dug a moat around my house that is at least 4 blocks wide and filled it with lava. (It doesn't need to be filled with lava for it to work. Just the space is enough but then that wouldn't be as cool).

The reason why I didnt directly dug a moat against the corruption is because the moat would have to be really really wide to protect against the corruption. You have to be relatively far away from it. There is no distance away from hallowed land requirement. because of this, it is easier to buffer your home with the hallowed and then prevent that from spreading too far. Plus there are more resources the hallow can't spread through such as mud which the corruption is spread through.

Solution 3:

I know this is really late and you might have found the answer by now, but in case people in the future are looking for a fix for this, the doors to the rooms must be CLOSED in order for them to spawn in them. Otherwise, they won't spawn.