Nonplussed (2,3)
'nonplussed' is the definition.
(synonyms)
This is all the clue.
(Other definitions for at a loss that I've seen before include "Puzzled, bewildered" , "(Running) in deficit" , "how incompetent companies can operate" , "Puzzled - below cost" , "Puzzled and indecisive? No profit in it" .)
(Other definitions for at sea that I've seen before include "Worse things can happen here" , "Lost or confused - on the briny?" , "In confusion" , "stumped" , "disorientated" .)