problem with hot water been expelled....please help! thanks?

My combi boiler (model puma 80e) saddenly started expelling hot water with lot of pressure into the garden from the pipe I think is connected to the safety valve. Do anyone know the cause? After that appened, the water pressure went down to zero, so I tried to pressure it up again, but the water went straight out again. I never had problem before! What can I do to bring it back to normal? Many thanks.