Two things to check:
1 - there's a switch on the upper right side of the iPad (if you're holding it in the portrait position with the Home button on the bottom). Move the switch to the opposite position and see if that icon goes away and your iPad can now re-orientate.
...or...
2 - Double press the Home button(or swipe up with 4 or 5 fingers if multi gestures are enabled) so that the screen rises and you see the multi-tasking bar. Swipe to the right so the controls come into view from the left side. The leftmost icon is either an orientation switch or a mute button, depending on your settings. Press it and watch to see what a tiny pop-up tells you what you've done.
Both the side button and that icon can be controlled in your Settings app to be orientation lock or a mute button. You set one for one function, the other will have the opposite function. Go to your Settings app, and in General, You'll see a line titled "Use side switch to...". That's where you control the behavior.
Probably a lot more info than you need, but I hope some of it helps you. Good luck.
Marilyn