I made these three trees and after coming back to them the shading looks all weird and dark.
Does anyone know what happened?
We have no way of knowing- we don’t know how you created them, what shaders you have on them, how your scene is lit, or anything else 
To me it looks like you need stronger lights in your scene, but that is very literally a crapshoot. Please provide more information, it will really help you get the answers you’re looking for
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I dont think the lights have anything to do with it. I came back to this scene after about a week and then suddenly the lighting had changed
We still need to know more.
Which display mode is enables? Both Material Preview and Rendered have “Lightning” checkboxes. This settings are unique for each window/tab
Another possibility, selecting objects and entering Local view without lights might result in an unlit scene as well.
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Again, we have absolutely no information that will help us help you here
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Things generally don’t change on their own (excepting possibly automatic graphics card driver updates, but they don’t usually result in this). Which means you did something. But you might have done it inadvertently; that’s incredibly common, especially as a Blender beginner, but it happens to veterans too – except they have more experience in diagnosing it. I can’t count how many times I pressed a wrong key without looking at the keyboard. And it did something behind the scenes, so I didn’t even notice to undo it. And then some operations later something was weird, and I had no idea, since I hadn’t consciously done anything.
There’s definitely something off with the lighting here. You might have turned the scene lights off, or the HDRI, set the HDRI strength very low, or set the scene to use an HDRI but have none connected, or set your lights invisible, or… Could be a lot of things.
So, look at everything that has to do with lighting. You’ll likely find some corners of Blender you didn’t know existed.
If you can’t find it, share your .blend file.
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Gremlins… they are real…
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Indeed, I recently hit a wrong key and a grease pencil object from a random file that I’d made weeks ago suddenly appeared in my scene.
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