Hey Rowan,
Can you please give me some more details? Can you send a track file?
This would be the hypoxia simulation, which happens when:
A) The cockpit pressure is below about 25 kilopascals...is the canopy there, closed, and sealed?
B) You've run out of liquid oxygen LOX (which is highly unlikely, as the 5L bottle can last up to 30 hours depending on altitude.)
C) You've switched to your emergency supply, which will only last a few minutes depending on altitude, and have exhausted it without realizing it.
You say the oxygen indicator stays at zero, do you mean the LOX meter, or the flashing oxygen flow indicator?