Not sure what you've already tried. Have you locked it using the Side Switch then accidentally changed the side switch to permit Muting/Unmuting?
Settings > General > Use Side Switch to: Lock Rotation (make sure check box is here).
Then try sliding the side switch. Also try turning off the device by holding the power and home buttons, then sliding the red slider to the right (then repower).
Did all that again. Still doesn't rotate. Only other thing I noticed doesn't work is the volume increase button next to the side switch. The volume down button works, but not volume up. Not sure if this may be relative to the screen locked from rotating malfunction. Please advise on the next step. Thanks