Moderator
James Huff
(@macmanx)
Volunteer Moderator
At Users -> Your Profile, make sure that “Disable the visual editor when writing” is not checked.
If that is set properly, check https://wordpress.org/support/topic/wordpress-40-master-list to see if you match any of the know issues or conflicts.
If you don’t, are you using any sort of caching plugin, or do you see a “Clear/Purge Cache” button in your Dashboard or admin bar? If so, would you please try clearing the cache?
If that doesn’t work, try deactivating all plugins. If that resolves the issue, reactivate each one individually until you find the cause.
If that does not resolve the issue, try switching to the Twenty Fourteen theme to rule-out a theme-specific issue (theme functions can interfere like plugins).