chunks disappearing

I logged onto my single player world and noticed some chunks not loading. That being somewhat typical in Minecraft, I ignored it. I logged on later and experienced the same thing. The chunk not loading was an obsidian platform on a lava ocean. I placed a bucket of water next to it to see if it would load and it did not. Next, I tried placing cobblestone in the walls of the hole and they did not appear in the hole, but were still taken from my inventory. Finally I decided to jump in (after backing up my world of course) and found that I was caught bouncing on the edge of the obsidian. I relogged and found myself in a sky with no blocks at all in sight. the chunks will still not load in my world.