There'll be a pressure gauge on or near the boiler, depending on what type of boiler you have fitted. This indicates the pressure in the heating system. If this drops to 0bar or normally 0.5bar the boiler will not operate.
To top up the heating sytem, there'll be a filling valve and a non-return valve. this is normally connected by a flexible hose, or some boilers have internal filling valves.
Re-fitting the radiator shouldn't be a problem. Make sure heating is turned off, refit the radiator and open both radiator valves. You need to fill the boiler to about 1.5bar and vent the radiator. When the air stops or slows down you need to Keep topping the pressure up to about 1.5bar until water is present at the radiator vent.
When the radiator is full - again top the boiler up to between 1.0 and 1.5bar.
Check radiator for leaks, a leak will make the pressure drop over a period of time.
If the radiator you have removed is downstairs, you may need to vent all upstairs radiators too.
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