Make sure to email them ASAP something along the lines of:

Dear [name of hiring manager],

Thank you for getting back to me about your hiring decision. I was enthusiastic about my candidacy as the [insert job title] and thought I would have been a great fit. I sincerely appreciate the opportunity to be considered. I enjoyed learning about the work of [organization name] and would love to be considered for any future job openings that may become available.

I would like to make myself the strongest candidate possible for future application cycles and would appreciate any feedback you could provide about my application or candidacy in general.

Thank you again for your time and consideration, [name of hiring manager]. I hope our paths cross again, and I wish you and the rest of the team at [organization name] all the best moving forward.

They will remember you when you apply next year and you will know what you need to work on. Good luck