If you have an older model of iPad 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 screen rotation - or muting the iPad:
Settings > General > [Use Side Switch To:] - select either Lock Rotation or Mute
For newer models of iPad without the side-switch, running recent versions of iOS/iPadOD - to enable or disable Screen Rotation, you need to access the Control Centre. Swipe downwards from the top-right corner of the screen - then tap the Screen Rotation lock icon:
If screen rotation proves to be problem with some Apps - and not others - you need to be aware that not all Apps are coded to operate in both Portrait and Landscape screen modes. Apps that have a fixed screen orientation will always appear in the orientation determined by the App Developer.
I hope this information and insight proves to be helpful