i cannot switch off my ipad air

Reset your device:

Device Reset (won't affect settings/data/music/apps/etc)

1. Press and hold (& continue to hold) BOTH the Sleep/Wake button & the Home button.

2. Continue to hold BOTH (ignoring any other messages that may show) until you see the Apple logo on the screen.

3. Release BOTH buttons when you see the Apple logo and allow the device to boot normally.

Try giving it a reset. Hold down the sleep and home keys for about 20 seconds. You should see a silver apple, which means it'll reboot and get unstuck.

The button should click and do something (put the device to sleep or power it off). If the sleep/wake button does not "compress", click or give any indication of moving, then it may be a hardware issue. Being that it's an iPad Air, it's of course still under warranty. I would make a reservation (required along with your Apple ID credentials) at an Apple Store's Genius Bar and have them evaluate it (for free) and replace it if it's in fact defective. Before taking it in, be sure to back it up so setting up the replacement device will be much easier than starting from scratch.

An option for you is to use Assistive Touch until you can get the device checked out by Apple:

Enabling AssistiveTouch floats a virtual button on-screen which can mimic all hardware buttons on the device:

Settings>General>Accessibility>AssistiveTouch: turn ON

*Note* If you power off the device using the AssistiveTouch panel (tap the on-screen AssistiveTouch button, Device>Lock Screen (tap and hold to power off)), you will NOT be able to power on the device if your Sleep/Wake (power) button is not functioning. You will need to connect the powered off device either to a PC's USB port or an external charger to power the device back on.