Worcester Bosch boiler reset!!!?

Hi,
Hoping someone out there can help pretty sharpish!
My combo boiler seems to be playing up - I'm not able to get any hot water. I can still get heating fine. I noticed the water dial was at zero so I loosened the screw to fill it up, but it's still not working. I want to press the reset button - but the reset button doesn't appear to be a button, just some kind of solid hard rubber screw type thing!?!?