getTemplateVar has now been removed from the MODx framework so the method of getting the value has (obviously) changed.
You now need to do this for the current document:
$val = $modx->resource->getTVValue('name_of_tv');
$val = $modx->resource->getTVValue($id); // Id of the tv (not the document)
or
$resource = $modx->getObject('modResource',array('pagetitle'=>'SomePage'));
$val = $resource->getTVValue('name_of_tv');
$val = $resource->getTVValue($id); // Id of the tv (not the document)
Of this if you need the value from another document:
$thumbtv = $modx->getObject('modTemplateVar',array('name'=>'Thumbnail')); // Get the Template Variable 'Thumbnail'
$thumbraw = $thumbtv->getValue($row['id']); // Get the value for a particular id
$thumb = $thumbtv->renderOutput($row['id']); // Or get the output for a particular id
