Why did my dwellers become unhappy when I left the game unattended for an hour? [closed]

Death is the biggest deficit to happiness. As for what caused the death, it could be any number of things - especially over an hour's timeframe - multiple attacks and fires would certainly be enough to kill at least one dweller.

Additionally, as Nelson has already stated, if you don't have a Mr. Handy on the floor where you generate your food and water (you might have others in the vault - but they won't collect resources on other floors) then your resources won't be collected, and Starvation, dehydration, and loss of power can affect happiness as well.

Did you have any Mr. Handy?

If you don't, the resources do not collect and your vault will run down.