While experimenting with Git, I changed a file's content from 1 to 100, and was able to commit and see it in the log.  But after some more test and
git checkout <some_hash>      // I think this is to revert the whole repo to a 
                              //     certain state
git checkout master           // And this is to change the repo to the most 
                              //     current state
the commit histories were lost in git log.  What happened -- could the commit have gone into another branch and how to view it or get it back?
 
    