Very long shot I know... but my Nike HRM stopped working, regardless of whether I took the battery out, pressed all buttons to reset or what... turned out one of the buttons had got stuck in it's depressed state and had just frozen the whole thing.
If it were me i'd try applying a small amount leverage pressure at the casing join at various points around the unit to see if it gives any clues on how it's fixed together. If it has gone completely balls up though I'd probably end up just prizing it open anyway - nothing to lose! lol
