Minecraft looks like a bad texture pack

"fixing" a corrupt file for the purpose of minecraft is near impossible.

If you wish to try and salvage your save and read what is left:

1. change the extension of the document back to .dat
2. follow SevenSidedDie's advice and use NBT Explorer to parse the file

if NBT cannot read anything legible, the file may be beyond repair.

if NBT can read it, you can copy information from it to a new save (such as inventory items / settings)

I know this isn't excactly what you want to hear but I hope it helps, Good luck :)

Save, quit, and restart. The screen will be back to normal.