Is it stuck in 'portrait' or 'landscape' mode when you want it to be the other way around? Perhaps it just seemed that this problem arose suddenly, and you can't work out what you did to cause it.
There is a simple solution to this problem.
Control your Rotation Lock from the Control Center
You have inadvertently turned 'on' your 'rotation lock', and you most likely did this by accidentally tapping an option in your Control Center.
Look for the symbol shown here - a lock with an arrowed circle around it. This is your Rotation Lock.
White Background = 'On'; Grey Background = 'Off'
For any of the symbols shown on this screen, a white circle in the background indicates that the corresponding feature is turned on. A grey background indicates that the feature is turned off.
Just tap on the symbol to switch it from on to off and vice-versa.
So, for your Rotation Lock symbol that is showing a white background, tap it once to make it show a grey background - thereby indicating it is off. You will then find your screen is able to flip around as you change its orientation.
See the two images below for further explanation.