Taking a screen shot on iPad by using dialog box that I access on the screen. It went away!

Are you referring to the Assistive Touch button?

If so, go to Settings ➜ Accessibility ➜ Touch ➜ Assistive Touch, and make sure its turned on.

click here ➜ Use AssistiveTouch on your iPhone, iPad, or iPod touch - Apple Support

Perhaps you refer to Assistive Touch:

Settings > Accessibility > Assistive Touch > Assistive Touch - set to ON

There are many ways to trigger a screenshot:

Take a screenshot - Apple Support

In addition to using physical buttons, you can trigger a Screenshot with your finger using a corner-swipe gesture - this being far easier and quicker than using physical buttons or Assistive Touch:

iPadOS 16 and earlier

Settings > General > Gestures > 

• Allow Finger Swipe From Corner - set to ON
• Left Corner Swipe > select Screenshot or Quick Note as required
• Right Corner Swipe > select Screenshot or Quick Note as required

iPadOS 17

Settings > Multitasking & Gestures >

• Swipe Finger from Corner - set to ON
• Bottom Left Corner > select Screenshot or Quick Note as required
• Bottom Right Corner > select Screenshot or Quick Note as required

The corner swipe gestures are also available when using an Apple Pencil:

Settings > Apple Pencil > [Pencil Gestures] Left | Right Corner Swipe - select Screenshot for one, other or both gestures