For future reference, you can shut-down your iPad using a variety of methods - the easiest being from iPad settings:
Settings > General > Shut Down
Equally easy is to use the iPad Control Centre. Swipe-down from the top-right corner of the screen to expose the Control Centre - then tap the shut-down icon at top-right:

A little more awkward - you can use the physical buttons of the iPad to access the shut-down page. Simultaneously press both the top-button and either of the two volume-buttons - and then use the slider.
Finally, Siri can be asked to shut-down the iPad using the "Shut Down" voice command. To trigger Siri, simply press-and-hold the top-button until you hear a chime.