The "pop" you heard was the flaired end of the pin shearing off as you pushed out the pin. On modern narrow chains the pins are riveted at the ends to increase their strength and resistance to side loads. Once the pin is pushed out even half way, that flair is gone and the connection is much weaker if the pin is reused. That's why you never partially remove and reuse the same pin on any modern chain. Shimano sells specific replacement pins and KMC, SRAM, and Wippermann all use a master link.