No compression in one cylinder Whats the cause?

No compression in one cylinder can be caused by several issues, including a blown head gasket, a damaged piston or piston rings, or a problem with the valve mechanism such as burnt or stuck valves. These issues can lead to a loss of sealing within the cylinder, preventing it from building pressure during the compression stroke. It's important to diagnose the problem through a compression test and potentially a leak-down test to determine the exact cause.