Most boilers fail to start if they drop under 0.2 bar. To refill to 1 bar, look under boiler and find a slivery fexlible hose. It may look like a loop. on this pipe they could be two small valves with balck handles (or maybe just one) turn this handle and you should hear water rush into system. Keep an eye on the clock and turn of valve when needle gets to 1 bar.
With the radiator, vent it (unlikely)if air comes out besure and top up water in boiler to 1 bar.
more likely to be a shut off lock shield valve,. Make sure both sides of radiator is open (turn the same way as if opening a tap, Rightly tighty, lefty loosey) . If you are sure lock shield is open, remove the head of the TRV (thermostatic rad valve) you should be albe to do this by hand. When it is off you will see a metal pin, gently give it a little pull. Sometimes these stick.