Re: Cold radiator.
@Carol,
Do you have trv's as shown above? - as stated they can stick. Sometimes they can be freed off, but sometimes need replacing.
Otherwise, you may have sludge in that rad blocking the flow.
Sometimes it's just the trv head, which is an easy diy replacement. For the body, get someone in. If flow returns when the pin is pressed down, the fault is the head. If not it's either the body or sludge in the rad.