Minecraft – Will experience orbs burn in lava in minecraft

If the biomes shifted it's probably due to a corrupted "seed" (just a long number) which seems to happen quite easily when the server crash. If you have an old save that still is ok you can get the correct seed from that save, and then edit your current save and set the correct seed.

It's really easy to read and set seeds with LolEdit. You just open the save in the editor and modify the seed field.

If you have explored new areas while the seed was corrupted you might also get problems with "sliced walls". Check my slighty longish answer in this thread for a solution to that.

Minecraft's difficulty setting is an option; it's not reliant on your saved game.

When you load Minecraft and you're presented with the options to load Singleplayer or Multiplayer, click on Options.... Then, click on the Difficulty option until it cycles back to "Peaceful".

When you load your world, all the hostile monsters will disappear. If you later want to increase the difficulty, do the same thing, but cycle the difficulty to "Easy" or higher.

You can also do this after the world is loaded by pressing Esc to pause the game and clicking on Options... When setting the difficulty to "Peaceful" in-game, hostile mobs will instantly disappear when you return.