Without more context it's difficult to say what the best answer would be, but a simple "Thank you, I 'm glad I can help." Would probably be safest. If it's a woman, she may be looking for a compliment such as "That's not true, your great at (whatever she is asking you for help with)." Your answer would also be polite and acceptable.

Just ask "Why not? Why do you feel so helpless?"

It depends a lot on the context. Maybe they are just stating a fact? For example, "We need to move these boxes. They are too heavy for one person. When can you help? I can't do anything without you.'" But yes, maybe Nim is correct? Certainly it's possible and Nim's response about alternative ways to say this and how to respond are valid suggestions if you believe that the person is attracted to you and you want to respond in a positive way. But if you aren't sure what they mean, and don't want to risk embarrassing yourself, I would respond more cautiously with a question like "Really?" , as this is an invitation for them to explain what they mean. It's important to say "Really?" in the right time. Friendly and curious. Not too excited not too dismissive. You could add "What do you mean?" in the same tone.

Hi Bi Hana. Thank you for reaching out to me for help. If your question revolved around how to paraphrase "I can't do anything without you", I would suggest saying something a little less strong in meaning such as "we make a great team, don't we?" responding positively with "Yes, we certainly do!" On the other hand, if you want to paraphrase "That's what I should say", I would suggest saying something like a statement of acknowledgement/agreement. E.g. "Exactly!" delivering it with a genuine smile. It seems you have a workplace admirer! :)