I'd feel sad too. I love the birds that come to my small garden and miss the ones that suddenly stop coming for whatever reason. Hopefully you may have another nest before the season is over ...
If you go to WildlifeKate's web site (Google will get you there) she has several webcams, including one set up in a blue tit nesting box. They hatched only last week so plenty to see.
Well - they haven't gone after all. I hadn't seen the parents all day, and now they're working like mad. I tapped the side of the box as I thought it might provoke a few tweets but all was quiet. Perhaps they don't do so much feeding when it's so hot?
i also find it sad when they go,my box has been empty this spring but then yesterday i spotted a pair of great tits going in and out with beaks full of moss so later this year.i saw the jenny wrens fledge about 5 years ago,facinating to watch and they normally go early in the morning.