Before doing any repairs it might be a good idea to do these:

1. Place the watch to bright sunshine for one full day the dial facing straight to the sun.

2. After the sun treatment pull the knop fully out and keep it that way for 30 seconds (or more).

3. Press the knop back in and wait for 10 seconds.

4. If the watch now works normally, you saved the battery price and your fixing work…

5. If not, you unfortunately have to do as instructed here.

Good luck!