google.maps.event.trigger(map, 'resize');

http://www.digitalwks.com/blogs/diogo-raminhos/google-maps-v3-and-jquery-tabs-jquery-tabs-google-maps-bug/

If you are struggling against some problems with google maps and tabs be aware that, in order to avoid problems, jQuery sets the width & height of every hidden element to Zero.

To fix this problem all you have to do is trigger the resize event and the map will be working as it should.
To trigger the event all you need to do is

Google Maps v3 and JQuery Tabs (jQuery tabs google maps bug)

google, jquery

JavaScript