When I work with 2 split screen, one screen has a blank gray. I don't remember if there is a letter button that was pushed accidentally.
Who knows my friends all have a solution, Thank you very much :)
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Look at the top. It says "Back to Previous" -> That's how you get back.
What happened is that you accidentally hit Shift + Space Bar. This makes the current View go full screen. (The one your mouse is over) Alternatively you can just hit Shift + Space Bar again.
On the other hand, if you have trouble finding the geometry in your 3D view, you can either use Shift + C as pointed out by @HenrikD or, press "." or "," on your Num Pad to center the view on a selected object.