When i log on i can only see half the world. When i walk into the void i start slowly falling then i come back up to the land. I cant destroy or pick things up either

Put the summary of the bug you're having here

What I expected to happen was...:
I should be able to see the world and pick things up

What actually happened was...:
I couldn't see the world. I started slowly falling instead. I couldn't destroy or pick anything up. Also my iron door wouldn't open or would just stay open after pushing a button or stepping on a pressure plate.

Steps to Reproduce:
1. Log on
2. Attempt to play mine craft