Have you tried a factory reset? Press and hold MENU, CLOCK, and UP simultaneously. Select YES when prompted.
If it's been 15 months, I'm wondering whether you have changed the battery?
Had the same problem here. First, the “light” button stopped to work, then the watch started to flash the”please wait...” sign, and finally the clock stopped working at all. I replaced the battery, made the initial settings, but the only thing I couldn't configure was the size of the pool, which always showed the number zero. I tried to delete all my data records, but it also didn't work, the number zero kept showing up. I was only able to set the size of the pool when I forced the clock to reset to factory settings (Press and hold MENU, CLOCK, and UP simultaneously. Select YES when prompted.)
The reset worked for me have had my original Watch for 5 years but it developed a leak and the strap also disintegrated bought a new strap but clearly water was getting in so acquired a replacement form ebay had the Zero pool size problem. So came here tried the factory reset and hey bingo it worked also this enabled it to connect to my Ant Stick as the ebay one didn't come with one.