That's nothing. If the piece has not actually come off, leave it be. You could try prying it open enough to inject some glue, but that may give you mixed results. If it is loose then that is another matter, shim it open far enough that you can get glue through out the entire crack, then use a rubber band to hold the pieces together while the glue dries.
Once dry (let it sit for at least a day if not 2) then you can sand and finish it to your hearts content. You will end up with a line where you don't get finish because of the glue, but if you clean the joint carefully after applying glue it won't be that noticable.