In addition to the physical side buttons, the iPad audio Volume can be adjusted from the iPad Control Centre - or using Siri:
Adjust the volume on iPad - Apple Support
If the iPad must be turned-off (i.e, fully shut-down), this can be accomplished from iPad settings:
Settings > General > Shut Down
Assuming that your physical buttons have been damaged or are non-functional, your iPad may require repair:
iPad Repair & Service - Apple Support
If repair is required, you might be well advised to first have your iPad tested and assessed by an Apple Technician - either at the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP):
Genius Bar Reservation and Apple Support Options - Apple
Your nearest AASP may be much closer than your nearest Apple Store.