Why won’t my Airpods work?

I had to get a replacement Airpod because I lost one of mine. When I got the new one, I forgot the airpods on my Bluetooth. When I got them reconnected, the new one started working and the older one stopped working. Both airpods show up as “connected” on the charging widget but only one will play audio. And now that I’ve forgotten the device again to try to reconnect, they won’t reconnect. What should I do?