Great tit found dead in nesting box

I’ve been observing the two great tits in my garden for the past six weeks. They have both been very busy taking bits into the nesting box.  About five days ago I realised there was no activity. I’ve been watching all week and wondering what to do. Today I went to look in the nesting box and have found a dead adult bird but no eggs! I don’t know what to do and I wonder what has happened. Why are there no eggs? Can anyone help?