Hi and welcome to the Forum!
As eddieg has pointed out, you haven't inadvertently pushed the mute button which is above your volume controls? I'm not thinking it is this though as you say your volume slider is missing.
Did you notice this first occurring after using either Bluetooth or earphones directly in the jack? If you just pull the jack out or stop Bluetooth without first stopping the music you are listening to, this can happen. So try reattaching both of these and stop the music first and see if it makes a difference.
Otherwise do a reset.....this will not change anything on your iPad and is akin to rebooting a pc.
Hold the power off button and the home button simultaneously, keep holding them in and you may see the power off slide appear and disappear, wait until you see the Apple logo appear before releasing the buttons. It will take a little longer for your iPad to start. Let us know how you go.