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I suspect from the sound of it, that your heart monitor band may have gone bad. And need replacing. That would be unfortunate, but with the age of it, warranty no longer covers it, and failure is possible at that age.
One thing to try, is to reset the wrist band thoroughly, for at least 2 hours without batteries in it, then try again, as if it were a new device - but, a new device that has not yet been paired with the wrist portion.
After that, if it fails, replacing it may be your best option, unfortunate as it may seem.