Does the heart rate monitor give you strange values? Is it bouncing from low to high values?
This could be a problem with the heart rate monitor itself, however, most of the times the problem can be easily fixed.
Make sure to moisture the electrodes/pads on the chest strap before wearing the HRM. Sometimes dry electrodes cause this sort of problems.
The problem can also be caused by a depleted battery, try replacing it.
You can reset it. Check your heart rate monitor manual for more info on how to do it. In some HRM models this is how it is done. Take the battery out, turn it upside down and hold it in for 10 seconds. After that, put back the battery as usual.