If the page's editor only shows text (HTML) editor

instead of three tabs: Visual | Vamtam | Text , please follow the steps below:

1. Please go to Dashboard > Users > Your Profile (or use the shortcut "Edit My Profile" at the top right corner) and check if the option "Disable the visual editor when writing" in your user profile is unchecked.

2. Deactivate third party plugins to rule out any conflicts.