I saved every layer into one background and I don't know how to undo it. Please help ME!!

By the way, do you have access to Adobe InDesign? That would be a much more suitable application for creating a magazine. You still optimise your images in Photoshop but then place them in InDesign, where you add the text.

It is Photoshop CS6 and I don't know how to flattern it. Can you please tell me how

Layer > Flatten image will flatten all files to a single layer. You are asking to "unflatten" and there isn't a command for that.

Start by looking at Window > History. Can you step backwards to the point in time prior to flattening all layers into one layer? If you click the Open snapshot at the top, does that do it? Or did you save and close the file, which removes history?

The only way to "unflatten" a single layer file to to painstakingly select each object and jump it to a new layer with Ctrl/Cmd J.

Flattening our only copy of a file is a mistake most of us have only made once.

What software are you using? You've posted in The Lounge​ which is not specific to any software and isn't for support anyway. Let us know what you are using and a moderator can move your post to the right forum.

If you're talking about Photoshop, have you closed the document since you flattened it?