If you have upgraded to WordPress 3.3 / 3.3.1 and having problem with your Media Uploader, fear not. The culprit is wpTimeMachine. Deactivate it and you are good to go.
I am not sure if this is a serious bug or a feature in WordPress 3.0.4 but occasionally when I do an automatic upgrade of plugins, I will encounter with problems stating that my site is “Briefly unavailable for schedule maintenance”. When you encounter such message, you will no longer able to access the WordPress admin site and your blog.
When face with this problem, there is a quick fix for it as stated in Worpress’s Codex site:-
All you have to do, is SSH (sss <sitename> -l <username>) to your site and go to your default WordPress directory and remove the “. maintenance” file. If you are unfamiliar with Unix, you might not be able to see this hidden file when you do a listing of the directory using the command “ls”. Use “ls -a” instead to list all files including those hidden files. Once you are able to confirm that the file is in your directory listing, simply remove it by issuing the command “rm .maintenance”.
Your blog should function normally again and you can go back to your WordPress Admin site and try to do the plugin upgrade again.