CH Pressure relief valve 24i junior?

Have got a leak somewhere in th CH system and have to keep topping it up,couple of days ago must have put to mutch in as the pressure relief valve vented the system. The plastic lever that is located at the back right hand side is in the down position ie its not located back up into the locked position and i cant budge it. My concern is that with temps of - 8 and not going above freezing, have i got ice in the waste pipe that is keeping the pressure in the system?