There are lots of suggested ways to fix this problem. The simplest is to glue a toothpick to the inside of the saya with a little Elmers carpenters glue. I would not recommend training with it this way, but it should be plenty good to make it sound for transport and display. If the sword does not go all the way back in after the repair, don't force it. Just grab a metal nail file(not the sand covered paper ones lest the sand get loose and scratch up the sword) and sand it down a smidge at a time until the sword will seat all the way into the saya.
You should also look up some cleaning and oiling instructions if it is in fact a steel blade. Steel rusts if not properly maintained.
Breaking down bad habits,
and building new ones.