Why is my picture flattened?

I suspect the same thing. You probably created your elements on a Parent Page (formerly: Master Page) and after saving and reopening the content shows up on a document page.

For clarification we really need a screenshot with visible Page Panel or a sample document (without confidential data).

Content does not get "flatttened" in InDesign in the way you're describing. If  you see content on the page, it should be editable. Do you have content on a parent page and you're viewing your document pages? Are your layers locked?

If you could share screenshots of  your layout and layers panel, it would be helpful.

What is flattened?

• Layers in InDesign?
• Layers in linkes assetts?
• Layers in embedded assets?
• Exported PDF?

Are you sure it's original file?

You can only "destroy" contents if you convert text into curves - but you can't "flatten" contents into raster graphics by accident - you would need to export into PDF - or each page into image file - in a particular way to "flatten" everything, then import it back.

Can you share few pages?

...... 

Or you are talking not about InDesign file itself but about linked graphic that been flattened in Photoshop?