My inventory reset

I was playing in a beta survival world with a friend, and I had accumulated quite a bit of good resources. Recently my game has been crashing more often and I can't even join my friends' beta worlds anymore. On the rare occasions that I can join, I come to find out that all of my inventory had been wiped except for a starting map, and I am back at the inital spawn zone. If someone could find out what is causing this and how I can fix it that would be great.