Text disappears when I edit it
You highlight some text to edit it. The text box and the text both turn the same color, so the text becomes invisible.
Choose Format, Slide Color Scheme, click the Custom tab, double click the Background color patch and change the color to one that contrasts with your text color.
What's happening here?
When you select text for editing, PowerPoint uses a color that contrasts with the current background color to highlight the text.
But suppose your background color is black but you've added a white or light colored photo or graphic to the master. When you view the slide, it looks like it has a light background, but to PowerPoint, the background color is still black. Even if you've covered it up with other stuff, it's still lurking down there in ... well, in the background. So when you select your white text, PPT highlights it in white, rendering it invisible.