Push up firmly on the bottom of the piece of siding to reconnect the top lip of the loose piece of siding under the bottom lip of the piece of siding above the original loose piece. Pop the bottom lip of the original loose piece of siding back over top the piece of siding below it with the hook tool.
How do I pop my siding back in place?
https://www.youtube.com/watch?v=1rmvX1ZJDvU
How do you pop aluminum siding back in place?
https://www.youtube.com/watch?v=oWGw3YbeYHs
How do you fix aluminum siding that has come loose?