[Kevin]

Hi Tiffany,
I think Alan is right. I can see in the photo the non-working reed is much higher than any of the other reeds. You can tell by the way the cast shadow is longer and fuller. Too much of a gap and it lets air pass through the hole.
Another thing to check is if there is anything in the melodica that’s blocking the air-flow. It’s a fairly old model so there has been plenty of years for something to get in there. Insects,spiders,pennies cheese-puffs the list is endless.
When you press just that key do you hear air-flowing or does it feel like you are blowing into something solid (stopped-up)?