[Question] Git recovery with HEAD commit broken

I am not a developer for git, but i am a regular user with git, i came
the following problem:

A power off cause my top commit broken, and then git
branch/log/reflog..etc won't work.
I do a hack that i change the HEAD commit to the one that i can make
sure is right, and then
i do:

# git reset --hard HEAD

In fact, i hope git fsck can fix up such problems(maybe can backup top
commit for example)...
Someone has faced such problems or have some suggestion for this?

Thanks,
Wang