when I put any object it goes away . when I try to put it , I put it above thing not in the sky

Woah,

Thats crazy… I wonder whats going on

Have you got any terrain, grid plane or anything else in a normal position ?

Maybe check Disable Editor Cell Snap

Manually type in 0 in the Z Location box

Maybe drop in a grid plane, manually scale it to something like 16,16, 1 and location 0,0,0, see if you can place objects on it ???