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