Minecraft Biome Modification?

One possible reason could be data corruption. The Random Seed of each map is saved in the file, and this seed determines how the world looks. Due to data corruption during the crashes it could be that this seed has changed, which would effectively screw everything (but would deliver interesting results for sure).

Another possible explanation is that you either got a completely new map¹ or chunks got corrupted/replaced/lost.

Anyway, my best advice is to restore from backup. Even if you change the seed back (given that you know the original one) or find the corrupted chunks, there's not much to gain. Parts of the world are already corrupted and you could only remove them and let them regenerate.

¹: Don't laugh, that happened to me once...took me ten minutes to figure that out.

I'm hosting a SMP server and also had big problems with moving biomes and spawn points. When exploring new areas the map also got "sliced", where you had massive completeley straight walls where different terrains met.

As @Bobby hinted, this was all caused by a corrupted seed and here's how I solved it.

What you need:

• The original seed. (For this you need an old uncorrupted save and Loledit, see below)
• Loledit- For setting and getting seeds
• MCEdit- For deleting corrupted map data.
• MCMap- Renders an isometric map of your save (optional)

What to do (see the documentation for each program for more details):

1. Backup all your data!!!
2. Open the old save file in Loledit and get the original seed. For me on Windows7 Loledit shows an error message when starting, but I just press continue.
3. Open the corrupted save in Loledit and set the original seed.
4. Biomes are now back to their original layout.

Now the seed problem is fixed so the biomes should be back to normal and any new areas explored will be ok. All areas explored while the seed was corrupted are still broken and will result in the dreaded "sliced walls". This can be fixed with MCEdit if you don't mind losing any buildings in the corrupted sectors.

1. First you need to know which areas are corrupted. You can see it in MCEdit but I found it easier to generate an overview map with MCMap. The areas that are based on the corrupted seed stand out quite well and should be easily spotted in the generated map.
2. Open the corrupted save in MCEdit. The controls take some getting used to but are well documented. "Q" for flying up to get a good overview, then WASD to move around, right mouse button to move camera.
3. First use the select tool (leftmost button in button bar) to select any corrupted area, then press "select chunks" to snap the selection to chunks.
4. With the selection still active, go to chunk control (rightmost button) and press "delete". Rinse and repeat, select/delete all corrupted areas.
5. Areas are deleted as soon as you press delete so no saving is needed. You can also use MCEdit to reset the bugged spawn point to anywhere you like. Quit MCEdit before opening the save anywhere else!!!
6. Load the fixed save game as a single player map since it seems some data must be regenerated. After that you can open it again in Loledit to verify that the seed is still correct.
7. Finally finished. No sliced terrain and biomes back to normal. Enjoy your spanking new save file! :)