I spent too much time finding potential solutions to a problem a lot of us seem to be having so here's the solution that worked for me. I hope it works for you too!
After upgrading to 2.7 I lost my ability to add a post or edit an old post. The tags were missing on the editor.
I disabled all plugins (after spending hours trying everything else) and tested the editor and it was fine.
I then started activating plugins a few at a time and narrowed it down to a conflict with one plugin that was causing the editor to malfunction. It turned out to be Viper Video Quicktags. Which makes sense.
I deleted all Viper Video Quicktag folder and files and uploaded a fresh copy and all is good again in blogworld.
Good luck and Happy New Year!