on a particular page the visual editor is not showing. just the html. how do I get the visual tab to show back up?

Does this happen for one user or all, try creating a second user and see what happens.
All other pages are ok?
did you check the setting dashboard->users->author&user->edit (the user in question)->Visual Editor - Disable the visual editor when writing

its only one page. its happened on multiple websites. the visual editor is not disabled. happens for more than one user.

What if you swap to the default theme?
It could be a php memory issue, if you have a lot of plugins, try disabling ome and see if it works

what version of WP do you use?
as I remember, this is a WP issue. Check these links:
• http://wordpress.org/support/topic/278484
• http://wordpress.org/support/topic/279476