when you put item in showcase it's ok they stay in, but when u go out of my home and return in later all item are on the floor or are disappeared

Save your game (in case something vanishes or gets damaged) drop the items you want to display on the floor, leave the house or go down to the basemant and return to put your your dropped items in the display case most items will stay using that method. This will not prevent bugged items vanishing, or weapons falling through the display case which is a seperate bug. Anything that falls through a display case you should test on a weapon plaque or weapon rack to make sure its display properties were not altered. If it does not display right after falling through, it never will, go back to your save game and remember not to ever put that weapon type in display cases again.

If it was Keening that vanished it could be anywhere in the house, that thing likes to move, as does one of those dragon claws, the emerald one I think it was. The wooden mask vanishes, do not ever try to display it anywhere, leave it in storage.