That should be easy to glue and reclamp. As has been said here or AK, clean any crap out of the joint using maybe a sheet rock knife (sometimes the blunt back side works best), you can wedge up the joint with door shims to get better access. To glue that up, just squirt carpenters glue in there, using a squeegie (maybe a cake spatula) near where it is still bonded to force into the crack, and find some pipe clamps, and wood standoff blocks to cinch it back together. If you have to buy clamps, they sell 1/2" kits which just require one 10' long "black" pipe which you cut in half for two 5' clamps (you only need one end threaded. They should have them at a hardware super store like Menards. I wonder if the triangular glue blocks inside along that seam have also let go.