File a service request for repair/replacement service. It could be a software glitch (and you might want to look up information on how to do a factory reset for your specific device if you can't use the device interface - in general, if you connect to a desktop computer, you can force a reset by deleting all of the files from the partition that gets mounted when you connect the watch, but you should check to see if this will work for your specific device before trying it, because you don't want to brick your watch), but it's more likely a hardware problem with a physical or touch-sensitive button.
