why do only some of my radiators come on?

Hi, any suggestions would be grateful.
Our central heating has been working fine til today, when i got up i found the radiators were cold, even tho the system is on,,,i "clicked" up the wall thermostat to see if it would go on,,,it didnt :-(
so i tried putting the hot water on, it worked. after the water was hot i switched the slider off and heard the boiler fire up for the heating, out of 9 rads only 3 came on, any ideas please? it all worked ok yesterday,
thanks
gina
ps, my partner went up to the loft to check the small tank, he said it was below the water level so he pushed down the bullcock til the level was reached,,( i really miss the heating)!!! lol