On one of my subsites webpart editing and creation is no longer
possible.

My other SharePoint sites in the same site collection are fine!

When I try to edit or add a webpart on this one problematic site, the
WebPart control pane opens, but no controls appear-the pane is blank.
When I attempt to add a new webpart--- no webparts are portrayed

Again--- 'peer level' sites are fine.

Has anyone seen this before?