Is your boiler working? if yes then there is pressure in the system and it will be a faulty guage. If it does not work then it may be that the pressure is too low and you have a leak in your system. If the pressure was too high then your pressure relief valve would blow. (Probably looks like a red cap in your boiler, or a bent copper pipe on your outside wall) If no signs of that then there is a good chance you may have a leak somewhere you cant see it on your radiator pipework, so I would get it looked at.
Delboy
January 2009