preview changes and update don't work

I am trying to add a sentence to my page. ‘Preview changes’ works fine before I start editing. But once I have edited and I press the ‘preview changes’, nothing happens. When I select ‘update’, it clears my changes and displays the unedited version. I have tried clearing my cache and temp files – still it doesn’t work. As far as I know nothing has changed. I have used both the admin and non admin logins. I am not technical so I don’t mess about with anything. The only thing I did prior to editing the page was added a couple tags.