Minecraft save game problem?

I have been playing minecraft single player for a while now, but not to long ago i paused the game and came back in the room to see my computer on bluescreen. I rebooted my computer and everything still worked. But a couple of days later I tried to run minecraft and it seems to make new worlds, but when I try to run that same savegame it takes forever on the Building terrain screen. Then after that it takes forever to load the blocks, and it basically just goes nonresponsive. I never can even get my character to move.