at a loss

1. When you are at a loss, you do not know what to do or how to behave.
   I was completely at a loss because I forgot what I was supposed to say, and I knew that it was something very important.

   I was at a loss because my teacher told me to follow her instructions and then she forgot to give me the instructions.

   I met my teacher in the middle of the night. I was at a loss because I didn’t know how to greet him. Clearly, “good night” would not have worked. :-)