Looking at it in Firebug, there's an error in line 9 of jquery.plus1.js:

jQuery('.' + Drupal.settings.plus1.widget_class, context).each(function () {

The error is given as "Drupal.settings.plus1 is undefined".

I would suggest posting in the Issue queue for that module.

Date: Tue, 9 Aug 2011 15:01:55 +0100 From: Chris G cl@isbd.net Subject: [support] Why would an upgrade to the Plus_1 module mess up        some (javascript?) pull-down menus? To: support@drupal.org Message-ID: 20110809140155.GA5646@chris Content-Type: text/plain; charset=us-ascii

I have been slowly working my way to finding which module update had caused this problem, finally found it today, updating the Plus_1 module from version 6.x-2.6 to version 6.x-2.7 breaks the pull down menus on this site.

The working site is:-    http://www.globaloptimists.org/

I have a copy, with Plus_1 updated to 6.x-2.7, at:-    http://globaloptimists.isbd.net/

As you can see the menus at the top are permanently pulled down on the updated site.

Can anyone suggest where I should be looking to see what is causing the problem?  It's not urgent as I'm quite happy to leave Plus_1 at version 6.x-2.6 for the present.

-- Chris Green