Shut down and whooshing of oxygen

I have a hand-me-down GoLox which didn't come with a user's manual. There IS a window on the back of it that is supposed to show fill level, but nothing registers. I've used the machine several times successfully, but last night it was under my chair at a concert and it began "roaring" and fogging up -- also quit dispensing oxygen through the cannula. This was long before the GoLox should have been empty.