Girls, A girl told me that she thinks about me a lot. What does it mean?

A girl that I have been best friends with for about a year and a half now texted me out of nowhere in the middle of the night "You know, I think about you a lot".

How do I respond? I'm so confused.

I'm not sure if getting closer can work out for us so I would want to just give an answer that doesn't make her think I have a crush on her, but I also don't want to give her an answer that makes her think I just friend-zoned her (Just in case I really don't know).

Girls, A girl told me that she thinks about me a lot. What does it mean?
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