Player disappeared in LAN world

It is SO WEIRD THAT I AND MY FRIEND can't see each other, we tried teleport to each other, and tried to punch. It is not working anymore.

{{ However, I can still see his "fishing line" while he was fishing. My right left hand in minecraft in FIRST-PERSON MODE can still be seen.}}

{{ NO matter how my friend or me open the lan world, we still can't see each other, except the MOBS, they can still seek me and attack on me. 
At last, all pressure plate I steped on, not work, they didn't sensed me.
*(Sorry for hard-understanding English)*}}