Unity GameObject disappears after turning it into a prefab - unity3d

I am making a 3d game in Unity for a school project and I have followed several online tutorials. I made a simple door in Blender, imported it to Unity, and placed it in the scene and it worked fine, but then after I made some minor changes to it and then placed the new finished door object into my assets, the door object "disappeared". The object is still there in the scene hierarchy but it does not show up in the scene or game view. Instead, it is just an outline if selected and is otherwise not visible. Outline of object
I looked online and could not find any similar occurrences and could not find any, so I tried unpacking the object (which I did not expect to do anything) as I am very new to Unity. I also tried placing the object into a new scene, and later a new project, but both did not work.

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Prefabs not visible in unity scene

I bought an asset from unity store and installed XR interaction toolkit therein. Everything works fine. When I drag prefabs from another asset into the scene and deploy it into oculus quest 2, these prefabs become invisible.
I have tried giving these prefabs the same properties as that of the visible objects in the scene but nothing is working for me. does anyone have an idea of what I'm doing wrong?
I also created a game object (cube) but it's not also visible when I deployed it. This suggests that the problem is not the prefabs that I'm trying to add to the scene

Objects disappearing from the hierarchy on new scene Unity

I am developing a game with Unity Collab in the last months, but always on one scene. Now that I want to create a new scene some things happen:
lighting settings are not shared (it is not a big deal because I can do them again)
every object that I add or copy from the other scene is appearing, but if I change the scene and go back here again there is nothing. My friends on collab actually see objects in it, and also the lights like the other scene but mine is always empty
What should I do?
Always empty like that

Unity deforms every prefab that I create

I have problem with Unity prefabs. Every time I create a nice looking UI in main scene and I want to store it as prefab for later usage if I open this prefab it's deformed(random cliping, moved UI elements in different directions, ... such a mess). If I try to fix this prefab in prefab editor to make it nice looking like before and then if I insert this back to scene it's totally unusable. It creates bigger mess than before in prefab editor... I don't know what to do. Editor throws no errors btw.
In first image is nice looking UI(example already created). In 2nd image is prefab editor. Text is totally out of panel and 3rd is from back in scene (means removed old one UI and place stored prefab from folder) the panel is gone.
Imgur - images are orded by scene to prefab editor and back to scene
On those images you can see a 3 elements... but if I make more complex UI, it's clipping through each other in prefab editor and also back in scene.
Unity version: 2019.1.7f1

How to replace model without a prefab file?

I'm new to ARCore and followed this article: https://haptic.al/arcore-101-fa6f93d4c003 to add a model with animation and see it with my phone camera.
I switched the model from the article (kitty) with a ball that I downloaded from Unity assets store, I just replaced the previous prefab object with the new one from the ball package that I imported.
Everything works fine, but now I want to download other model from the Unity store and I see that some models don't include the prefabs folder or a prefab file inside the the model package.
Is there another way to switch my current model with a new one without being use the prefab file?
Thank you.
Even if the downloaded file doesn't contain a prefab you can still create your own prefab by placing the object in the scene (.fbx, .obj files etc.) adding whatever materials you want it to have, and then dragging the name from the Hierarchy panel into the Project panel. The name in the hierarchy panel should turn blue if you've made a prefab successfully.
Note: you don't need a prefab to put something in the scene, a prefab represents an object that has a particular state saved instead of the defaults (transform rotation, materials, colliders, etc.). If you add things to an object and save that object as a prefab then whenever you place that prefab into the scene it will have those components still attached.

Unity3d saved as prefab, readding removes meshes

In Unity3d I found the feature of saving GameObjects through dragging them from the Hierarchy into the Assets Folder. I'm using this to test stuff in a different scene.
Strangely, I it doesn't work with this one properly:
after readding it, all meshes are missing:
Do you know how to fix this?
Is there a possibility to move gameObjects, maybe through code, from one scene to another one(in editor)?
It is possible to move GameObject from one scene to another by using DontDestroyOnLoad, if you need to do so in runtime.