If you are not running one of the browsers and versions listed at http://browsehappy.com/ please try upgrading or switching first.
Note: If you are using an IE9 browser in compatibility mode that makes it render as IE7 and IE7 is not supported. To disable see > http://www.sevenforums.com/tutorials/1196-internet-explorer-compatibility-view-turn-off.html
If you are running one of the browsers and versions listed at http://browsehappy.com please do these four things:
1. Try clearing your browser’s cache:
2. Make sure that you have third-party cookies enabled: http://en.support.wordpress.com/third-party-cookies/
3. Try with all browser extensions or add-ons temporarily disabled.
4. Try enabling HTTPS: http://en.support.wordpress.com/https/
If it’s still happening, let us know which browser (and version of it) you’re using and post any error reports you get.
You may be able to recover an earlier post revision. Go to Posts > All Posts, hover over the title of the post, click Edit and check the Post Revisions module for previous saved versions of the post. http://en.support.wordpress.com/posts/post-revisions/
If you're not seeing the Revisions module while on the editor screen, click Screen Options (top right corner of your Admin page) http://en.support.wordpress.com/screen-options/ to activate it. Then select the post and page modules to include them. http://en.support.wordpress.com/modules/#list-of-modules Then you ought to be able to go here > Dashboard > All Posts > Posts and locate the post and recover it from post revisions.
If you cannot locate the post for locating and restoring a post revision, and it's not in your Trash folder http://en.support.wordpress.com/trash/ then you will not be able to recover it. http://en.support.wordpress.com/trash/#restoring-from-trash