The radiator in one room is cold while the others are normal. What should I do?

I had exactly the same situation at home - the radiator in my bedroom was barely warm, while everything was fine in the other rooms. It turned out that air had accumulated in the radiator, and it helped me to just open the Mayevsky valve (the valve on the radiator) and let the air out until water started coming out. If that doesn't help, you should check whether the shutoff valve on the line leading to the radiator is closed or whether the system is clogged with dirt - sometimes you need to flush the radiator or call your building management to check the circulation of coolant throughout the entire heating system.