page parent error message

I’m a novice; I have spent the usual amount of time trying to make my somewhat complex blog work anyhow, mostly by copying and trying things out.

Now I’m having a problem with organizing pages.

Earlier tonight only about half the existing pages showed up in the “Page Parent” pull down box.

I thought it might be a bug fixed in the latest upgrade, which I had been postponing, so I upgraded to 2.0.5 from 2.0.4, using the Changed Files Zip from Mark on Word Press, https://markjaquith.wordpress.com/2006/10/28/wordpress-205-upgrade-changed-files-zip-changes-diff-changed-files-list/

Then, for a few minutes, when I tried to edit an existing page and assign it a different page parent, all the existing pages were available in the pulldown. But when I tried to assign one, I got this error message:
Fatal error: Call to undefined function: maybe_serialize() in /home/savoring/public_html/wordpress/wp-admin/admin-functions.php on line 937

This function had worked perfectly the last time I used it, but that was months ago. I am using a Mac now, and was using an XP box then, in case that matters.

Any ideas about what I have done to bring this on, or how I can fix it?

With thanks –
Rona Roberts