Have you tried:
– switching to the Twenty Eleven theme to rule out any theme-specific problems.
– resetting the plugins folder by FTP or PhpMyAdmin. Sometimes, an apparently inactive plugin can still cause problems.
– re-uploading the wp-admin and wp-includes folders from a fresh download of WordPress.
– installing & activating
http://wordpress.org/extend/plugins/use-google-libraries/
Thanks, esmi. Yes, I tried switching to the TwentyEleven theme and that did not fix the problem. I also run another site (hosted by Dreamhost), so I switched that theme to TwentyTen and the Visual Editor toolbar appeared just fine. So I ruled out any problem with the TwentyTen theme itself.
I will try the other solutions you mentioned. Will re-uploading the wp-admin affect user logins?
Thank you, esmi!
Re-uploading the wp-admin and wp-includes resolved the problem. The Visual Editor toolbar is back.