You don your masks & establish communication - and then try your best to re-establish pressurisation.
If that fails - you communicte the loss of pressure the the cabin crew and make sure the rubber jungle has deployed.
From there on - it's no longer an emergency, but a time-limited scenario. However, the important actors in this stage of the drama - the flightdeck - have a looooooooooong time available before they run out of oxygen. Therefore - chill! Think & then act! Your CC and pax are not going to die on you for the first 10-15 min., so spend 30 seconds calming yourself down.
If you're overflying terrain that high
- you have a decompression escape route designed by your company & their performance engineers. So turn onto your escape route and follow the descent restrictions specified therein (it's 10.000 ft
or MOCA, btw) while carrying out your "emergency" descent drill, bearing in mind that the real emergency is over and no-one is gonna die, just as long as you don't panic.
Enjoy!
