Hi, my iPad have no any button to adjust the volume. So, what can I do?

You could have accidentally changed the volume rockers for the notification alerts. You can change this by going to setting<sounds, then under "Ringer and Alerts" there is an option called "change with buttons." Just tern this off and it should work if did happen the way I think it did.

I hope I helped!!!

Double tap the home button to reveal the row of recently used apps. Swipe that row to show the left most area and adjust volume there.