I cannot get water back into the CH system after draining down?

I drained down our CH system and can't refill it now. Its a traditional system with a expansion tank in the loft. There is water in the expansion tank and the ball valve is working correctly. I've tried bleeding the radiators, but theres no pressure to blow air out, I've tried with the pump on and off, without success, with radiators open and closed. Water presure in the rest of the house is fine.