During a wellbore service in the winter months, a high-pressure gas release occurred. The work plan included pumping down the kill line when the downhole operations were completed. The kill line valves appeared in the closed direction, and no movement was found. This, along with the valve handles being absent, led the workers to believe the valves were closed. The kill line was pressure tested and a valve began to leak. Operations were shut down to change the valve at which point an uncontrolled pressure release occurred. The investigation revealed that the valves had been stuck open by ice. No workers were injured.