at a loss

Puzzled, perplexed, in a state of uncertainty, as in When his letters were returned unopened, John was at a loss as to what to do next . This usage was originally applied to hounds who had lost the scent or track of their prey. [Mid-1600s]

Below cost, as in The store was doing so badly that it was selling merchandise at a loss .

at a loss for words . Unable or uncertain as to what to say. For example, Father's tirade left us all at a loss for words . [Late 1600s]