Powerlessly is the adverb: you do something — or more often you watch, wait, or stand — in a way that shows you cannot stop or control it. The adjective powerless names the state; this word names how the action happens. It often sits with verbs like watch, look on, wait, stand by, because the picture is of a person who sees the thing unfold and cannot change it. Fairly formal; in casual talk people may just say I couldn’t do anything. It does not mean “without electricity.”