First you want to blow no harder than it takes to sound your notes. You can hurt the reeds when blowing too hard. Second please tell us more about your melodica. Is it new and the tone has never played?
It’s unlikely to be broken – probably one of 2 things:
Try blowing very very softly, and see if the note sounds. If it does, then the tip of the reed is too close to the reed plate, and needs adjusting. This is called gapping. You have to take your instrument apart to do this. You can search on this forum or the internet for more info on how to adjust the reed – its not difficult, and does need to be done from time to time.
If blowing softly doesn’t work, there may be something caught in the reed. It could just be moisture. First try a quick succession of blowing in and out in tiny breaths to see if that works. If not, try rinsing the inside of the instrument by pouring water down the mouthpiece, and letting it drain through the moisture release button on the end. It doesn’t do any harm, and you could even add some vinegar to deodorise the instrument. If so, rinse this out as well.
If that doesn’t help, you’ll have to open the melodica, and “ping” the reed a few times, to dislodge whatever’s there.
Please report back with any issues, I’m happy to help
I found a video which you might find helpful. The melodica in the video has the opposite problem – the note only sounds when blowing hard, meaning the gap between the reed (the tiny piece of thin metal), and the reed plate (the big piece of metal that are all the tiny pieces are attached to) is too big. Here they are closing the gap.
Your gap is too small, so you have to open it a bit. You need to work on the first reed – the big one nearest to the mouthpiece. Open it carefully until it looks similar to the ones next to it.