Possible Duplicate:
Git non-fast-forward rejected

How to fix this error? It didn't happen previously
Possible Duplicate:
Git non-fast-forward rejected

How to fix this error? It didn't happen previously
 
    
     
    
    git pull origin master will grab the remote's changes and merge them with yours. 
git rebase origin/master will replay you local changes not in the remote on top of any changes on remote. After you do a git fetch origin, pulling automatically does the fetch step. Learn about rebasing in git. 
What happened is that someone else (or you on a different machine) also pushed to the origin repo with a different set of commits.  Until you have those commits, it won't let you push unless you include --force. Don't do that unless you have to by the way.
 
    
    Well the message tells you what to do: git pull, (fetches and merges changes), then git push again.
