Minecraft – Will the Minecraft 1.1 world ever contain a Jungle biome

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.

Biomes are generated from the level seed stored in the level.dat file. This is one of the reasons you can visit maps from prior versions and find biome-like coloring (and why expanding the map will generate biome terrain).

The chunk terrain, however, is saved, so areas that were sand will still be sand regardless of what "biome" the game thinks it's in. Terrain already generated won't change, so if the level seed changes, you can find a grassy or snowy area next to a desert along the boundaries of the chunks.