My iphone is no longer going from vertical to horizontal mode

You may have the orientation lock set to ON. Double check and set the orientation lock to OFF. Please read the following:

Lock or unlock the screen orientation

You can lock the screen orientation so that it doesn’t change when you rotate iPhone.

👉 Open Control Center, then tap .

When the screen orientation is locked,  appears in the status bar (on supported models).

Axel F.

Open Control Center. Depending on your iPhone model and tap on the Orientation Lock icon. If gray it is off, if it is white and you see the red lock, it is on. Tap it again to turn it off. Access Control Center on your iPhone and iPod touch - Apple Support

Use and customize Control Center on iPhone - Apple Support

did you lock the orientation in Control Center?

Change or lock the screen orientation on iPhone - Apple Support

Rotate the screen on your iPhone or iPod touch - Apple Support

maybe try a force restart..

Force restart iPhone - Apple Support

Have you tried a restart? You might also try a Forced Restart...

Force restart iPhone - Apple Support (CA)

You are welcome. Glad things worked out for you.

Axel F.