How does it feel seeing a person you once liked and never actually forgot after a long, long time?

I need some people to share this with me because no. 1, I've never actually experienced it, and no. 2 being the fact that I need to read/see for myself the genuine feeling of being in this exact same scenario. Currently trying to write a book and this is one of the major turning points, so I'd like to hear some of your experiences in detail.

I'd also like to note that if they forgot about you (especially if you once dated them), please do share your reaction to that as well. Thank you in advance!