That's nasty. Impossible to see without knowing more. A geek might be required. I have something to check, though. I'm guessing you have bad code in your page (non-html, etc.)
I was going to say try duplicating the Page and seeing if you can edit that, but that's my other system, MODx. :)
Quickest idea: make a new page with the same content. If the new page is good, kill the first one, and rename the new one to replace it. btw, don't directly paste from Word or similar things, use the "Paste from Word" button for stuff like that. Beyond that....
I had a client who would paste God-only-knows what kind of stuff from Word, silly ad site code, or wherever. When you paste directly from Word, without using the helpful "Paste from Word" button, you get all kinds of nasty extra code in your post. In his case, it killed his site.
I would suggest going into the database itself, via PHPAdmin or whatever database interface your host has. The table you'll be looking for is WP_POSTS, and you'll need to hunt down the offending one and look at the post_content. If it's full of weird crap, you might want to save that, then delete it in the database so the page will be clean for editing.
I'm guessing they never gave you a host log-in, and the above might all be gibberish. In that case, geek definitely required. That's my best guess.
Maybe someone else will have an idea.
Good luck, Dave