If your iPad has been turned off, you'll need to press and hold the sleep button (the small one on the top (top being if you have the iPad in portrait mode with the round button at the bottom).
If that doesn't start it, try a reboot: Hold both the power and home buttons until the screen goes dark and the Apple logo then appears (the slide to unlock screen may or may not appear). Once the logo appears, release the buttons. After a time, your homescreen will appear. Please note that it may take some time for the Apple logo to appear or for your homescreen to appear (or both). This is normal. Eventually, the homescreen will appear.
That fixes a surprisingly large amount of iPad "errors." Let us know how you get on and good luck.
Marilyn