while pulling into my git repository from our central server, this error occured:
$ git pull origin master
* branch master -> FETCH_HEAD
error: unable to resolve reference ORIG_HEAD: No such file or directory
fatal: Cannot lock the ref 'ORIG_HEAD'.
I had this problem, and I solved it by removing the file .git/ORIG_HEAD then doing the pull again. The .git/ORIG_HEAD file was 0 bytes instead of the git reference it was supposed to contain, so I just got rid of it.