Problem refilling CH system after fitting Grundfos 15-50 130 selectric pum?

I have had to drain my CH system to fit a new radiator. Problem occurs when refilling.It only takes about 25% of the normal amount of water to fill it before it backs up to the header tank. There seems to be no water getting to the manifold and to the radiators via the smallbore pipework. Have tried firing up the sytem briefly to see if pump will circulate the water but no go.Hot water only seems to be reaching the pump and no further. Is there a switch or a valve on the pump that has locked off and can it be reset?