How to fix: Orientation Lock Icon Missing on iPad

If you have an iPad model with a physical side-switch, this is likely to be toggled “on”. Switch-off the side-switch and screen rotation should be fully restored.

The behaviour of the side-switch is configurable between locking scree rotation - or muting the iPad:

Settings > General > [Use Side Switch To:] - select either Lock Rotation or Mute

Another possibility, for newer iPad models, is that you have disabled Screen Rotation in the Control Centre. Drag you finger downwards from the top-right corner of your iPad screen to reveal the Control Centre. Tap the Rotation Lock icon (here illustrated in “rotation locked” setting):