B-07 WHAT HAPPENED TO GOKU'STAIL?
Goku's tail was cut off by Pooaru early on in the series. It later grew back during a Tenkaichi Budokai. This newly grown tail also came off when Son Gohan (Goku's grandfather) tugged it too hard on the one day he was able to come back to earth. It grew back once again, during his own training between then and the next Tenkaichi Budokai.
When Goku went to train under Kami-sama, Kami-sama wanted to restore earth's moon, so he permanently removed Goku's tail to avoid any more problems.