I have seen a similar event twice in my career.
On the first occasion the Auto function of the pressure controller had failed and due to a combination of crap weather and a heavy ATC environment this wasn't noticed until the Cabin Hi Alt light came on. Selecting Manual pressurisation quickly solved the problem.
On the other occasion the F/E quite simply forgot to put the packs on after take-off!
In neither of these occasions was there a loss of cabin pressure for the cabin had climbed at the same rate as the aircraft so there was no pressure to lose.
Likewise the cabin altitude was exactly the same as the aircraft altitude.