Was contacting my ex a mistake?

I was at work today, and was totally thrown off when I ran into one of my exes from high school. He came through my register, and he seemed to recognize me, but didn't say anything regarding it. After he left I began feeling tempted to reach out to him. We went our separate ways on bad terms. He tried to contact me a few years back, but I was rather rude, because I was still upset by how he had broken up with me. Today I made the decision to send him a message on Instagram, asking him if he recognized me and telling him that I hoped all was well. He has yet to respond and I'm starting to worry. Was messaging him a mistake? We dated when I was 15 and he was maybe 14. I'm now 19. Help? How long should I give him to respond before I should just accept that he's not interested in talking.
Was contacting my ex a mistake?
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