When the battery is not functioning, the meter is designed to sense this and just won't turn on. There is also a battery-check button. But I guess you could always try warming up the battery in your pocket first. Another thing to check is if you dial is actually coupled to the meter
by the pin in the right position - this might have not been engaged if you didn't do it when you first attached the prism. I guess if your perimeter gaskets were worn out on the meter, there's a remote possibility of light leaking in through the sides. It's a good idea to double check your meter from time to time anyway using something like a spotmeter and gray card. Like any other kind of meter, they might need calibrating
by a pro from time to time.