Cause of Death ? Germany

Hey guys I sadly noticed that one of my hives die Not make it through Winter - Lots of dead bees on the entrance so I decided to open it up (kinda warm today) - what could be the cause of death ? Last varroa treatment was on 21.12.24

Hey guys I sadly noticed that one of my hives die Not make it through Winter - Lots of dead bees on the entrance so I decided to open it up (kinda warm today) - what could be the cause of death ? Last varroa treatment was on 21.12.24