when i play 1.0 on world all my stuff is gone

all my stuff is gone...when I wanted to try 1.0 beta on my world (that I play on with 1.7.4) all my stuff is gone! everything in my inventory in gone everything in my chests where gone now I have to start over with a house BUT NO THINGS even when I go back to 1.7.4 my stuff is still GONE!!! HELP ME!!