There are a couple of things to try. 1. Make certain your phone is not locked in a specific orientation by going to the control center. To do this, swipe up from the bottom of your screen. You will see an icon with a padlock and a circular arrow. Make certain that that is NOT highlighted (or locked) - It needs to be UNLOCKED. Click this a couple of times to turn it on and then off.
Then,
2. On your home screen, hold your phone in landscape mode before you try to open any app. if when you open the app, it still stays in portrait mode, shake your phone from side to side a few times. If there is a specific app that you are getting stuck with, report back here. There have been lots of reports about Facebook getting stuck and not opening in Landscape and staying stuck in portrait.
Please let us know!