Just before I begin playing MC, I
1. Drag the current copy (right-click-drag) of the world I plan to play (the world as it was before my last exploration) from the Desktop to C:\UserName\AppData\Roaming\minecraft\TempStorage (I created this directory).
2. Drag a copy (right-click-copy) of the world I plan to play (the current world as it exists now) from C:\UserName\AppData\Roaming\minecraft\saves to my Desktop (I have 2 emergency backups should I fall into lava),
3. Check and delete anything found in C:\UserName\AppData\Roaming\minecraft\crash-reports.
4. Check and delete everything with a date (YYYY-MM-DD-#.log.gz) found in C:\UserName\AppData\Roaming\minecraft\logs
5. Open Minecraft and explorer/build to my heart's content knowing that anything catastrophic can easily be undone!!!
Had you done that you could have restored (copied what is on the desktop) to C:\UserName\AppData\Roaming\minecraft\saves and lost nothing except the time it took you to fall into the lava,
I did not read 6 pages of replies, if a duplicate I apologize.