Yikes... That always hurts. Still, it depends on the situation. Are you dating them? Perhaps they aren't ready to say it back... In which case you should talk to them, and be understanding. If its not someone you are in a relationship with, then it could very well mean they don't feel the same way.
...Okay,I was one of those people who said "Thank you" to a couple of admirers-I was "into"in-appropriate Responses back in those days...because I was Dating and Uncertain How to Act...or What to Say back.I thought it was a Neutral Response, because I wasn't In Love with these guys;I just Liked them alot...and they wanted to get more serious.Didn't Mean to Hurt anyone...they just didn't Turn on my "Emotional"button.
I think you have won the battle, in first part. They were shocked to get your response, you said " I LOVED THEM AND THEIR RESPONSE WAS THANK YOU." Now they are confused whom do you actually love, they will let you know after they will decide who really loves you after selecting amongst them selves the ONE LUCKY GUY BY LOTTERY SYSTEM.
Don't act before trying too understand why they don't give you the reaction you was looking for. In my opinion you should sit down with the person and see we're their feelings are. Another thing find out if they have issue being self expressive due to past experience or rejection and broken hart could cause of to close in from allow anyone too close . So my advice communication key talk to em
This person could fear that you might not see who he is really or that he thinks there are things he thinks are unlovable about himself. Bottom line he worries that if you "really" knew him, like he were really bald or something, that you would change your mind. I know,of course, something that trivial wouldn't change your mind though as girls see beyond the physical things.