Motionlessly means “without moving at all” — the adverb form of motionless. It tells you how someone does something, not what they are: he waited motionlessly, she stared motionlessly at the door. The meaning is the same complete stop as motionless. Though actually, real English often prefers the adjective after the verb — he stood motionless — and skips the -ly form. Motionlessly is correct, but a little stiff, the kind of word you meet in writing more than in talk. If you use it, you are not wrong; you just sound like a book.