I had a rad which went cold throughout (top to bottom) in our landing 2 years ago. It was in need of bleeding (must have let out 20 litres of water and still no air coming out.
Gave up on it. Plumber advised it could be a kink in the piping and would need to cut and lift floorboards. Forgot about it until yesterday my toddler daughter touched it and said 'hot'.
She was right. It is now fixed with no intervention, 2 years late.
This is of no help to original poster but would be interested if anyone can explain the sudden cure