Is there a lock symbol at the top of the screen next to the battery indicator ? If there is, then depending on what you've got Settings > General > Use Side Switch To set to (rotation lock or mute notifications), then you can lock/unlock the rotation by the switch on the right hand side of the iPad above the volume switch, or via the taskbar : double-click the home button; slide from the left; and it's the icon far left; press home again to exit the taskbar. The function that isn't on the side switch is set via the taskbar instead : http://support.apple.com/kb/HT4085
If there isn't a lock symbol then try a reset : press and hold both the sleep and home buttons for about 10 to 15 seconds (ignore the red slider), after which the Apple logo should appear - you won't lose any content, it's the iPad equivalent of a reboot.
Check the Mode switch on the side of the iPad (just above the volume rocker). It is possible you have the iPad set to use that switch as the mute button, in whcih case the orientation control would be found in the app tray (double clikc the home button and swipe to the right in the tray. The control will be to the far left).
If that doesn't solve the issue, try a reset: Simultaneously hold down the Home and On buttons until the device shuts down. Ignore the off slider if it appears. Once shut down is complete, if it doesn't restart on it own, turn the device back on using the On button. In some cases it also helps to double click the Home button and close all apps from the tray before doing the reset.