"Hard" is an adjective and an adverb. "Hardly" is a different word and changes the meaning of the sentence completely.
"She works hard" means she puts a lot of effort into her work.
"She hardly works" means that she barely works, she works almost not at all.
One of the most annoying of co-workers is the guy who comes to your office and says "Hey Dave, are you working hard or hardly working?"