Notes:
Radio is not a permanent tab like the other music tabs, it only shows when it is available, will disappear when it is not.
It might not be available if: A) you are in a dead cell data zone, and it's switching songs. (Maybe also if data connection is not 3G or better). B) wifi only idevice? Have to be connected to wifi. C) It can also disappear if there is a server issue (happened a couple times w/beta). D) If you sign out of your Apple ID/iTunes account.
Instead of rebooting your idevice, you could close the app by going to the multitasker and swipe it up and away. Then make sure you have wifi or cell data flow, open Music app again and it should appear.