Text Disappears in InDesign When Changing Margin Sizes

What's your page size, what's your margin size? What's your paragraph styles setup like? Is it all text - any right indent margins? Any non-breaking spaces? Any no breaking characters? Any no-break applied? Any tables? 

Very hard to know without seeing it. It could be 1 line of text not able to anywhere.

I'd start by opening the Story Editor you can see the overset text

Delete some text until it appears 
If it doesn't - delete some more

Once  you have the text that is causing the issue you can examine that text to see why it won't flow.