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 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.