My seven year old zebra finch died. What could have killed him?

Hi. I bought two beautiful zebra finches seven months ago. Recently, I left for two weeks and they were being looked after by my mum and cousin. I was in the Czech Republic when my mum told me that the white boy zebra finch was ill. He couldn;t raise his wings to fly up and sat on the floor all evening. When he did fly up to get some water he simply fell off as he didn't have enough power to hold on. Later in the night my mum took him out of the cage and his strength was so low he wouldn't move or try and fly away. My mum tried everything. Feeding him, stroking him, putting luke warm water on his feathers to revive him but he died at 2am in the morning. The thing that most confuses me is the fact that he was the most active and happy bird and never ever had any signs of illness prior to his death. What could it have been. Any suggestions. Thank you.