Thank you, esmi!
Re-uploading the wp-admin and wp-includes resolved the problem. The Visual Editor toolbar is back.
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?
