Want a cheap fix?
Pull it straight back, off the control shaft.
It should be obvious what failed looking in the hole on the backside of the control knob.
Regardless, squeeze some RTV or similar adhesive into the hole and slide it back into position. Don't go crazy with the amount, use only enough to fill it 3/4 full or so. You don't want a mess squeezing out and gluing things you don't want glued.
Let it cure and viola!
Using an adhesive like RTV will allow you to pry it off in the future should it need to come off to remove the radio. Most home improvement and hardware stores will carry RTV silicone adhesive. Ask if you need help finding it.