It should be pronounced 一声。In this context it means “feel at a loss what to say."大家好！
I recently heard the following on a TV show:
(Context: co-worker explaining why one of his co-workers suddenly left the room) 怎么说呢，一看见你这一美女大记者就发蒙了。别介意啊
Would anyone know what 发蒙 means and how to pronounce it in this context?