Baby blue tit in nest

Hello,

We have been lucky enough to witness a family of blue tits nesting in a hole in the wall underneath a window. It’s been great to listen to them and watch the parents busying around to feed their young. 
Today I feel like we said goodbye to our little blue tit family as there is absolutely no movement and the babies that fledged the nest recently have not returned like they had the last few days. So I decided to put an endoscope in the hole to see the nest. I found a very large, very well built nest with a fledgling unfortunately laid lifeless. It’s sad but as I understand it, they have many babies so having just 1 pass away before now is quite a success. However I am slightly concerned that this little baby will not only prevent the blue tits nesting again, it may also rot and cause a bad smell along with flys and maggots. Strangely, of the 2, I’m more concerned that the blue tits won’t return to nest again next year. Can someone advise me what to do please?