If your git index for some reason becomes invalid, no need to worry.
Your index is corrupt when you see this error running usual git commands like git pull, git status, etc.:
error: bad index file sha1 signature
fatal: index file corrupt
Though it sounds bad, your changes are still there. Fix it by first removing the index file, then resetting the branch:
rm .git/index
git reset
You should be all good now.
To be safe, make a backup of .git/index before you delete it.
source link: http://makandracards.com/makandra/5899-how-to-fix-a-corrupt-git-index