This sounds like a steam system as opposed to a hot water type.
If I am correct do the following
1- The radiators that have been shut off should be opened to allow the water trapped inside to drain back to the boiler.
2- Make sure the automatic feeder is OFF if you have one.
3- Shut off the boiler emergency switch for now
4- Open a drain valve in the basement and let the system drain watching the sight glass or Tri cocks.
AFTER the water is drained to the proper level try the safety valve to make sure it is functioning.
Now open the valve to the automatic feeder Or had fill to the properly marked on your sight glass.
Turn on the electric power and watch the automatic feeder to make sure it does not flood the system again.
When you can get a qualified person to check out the system to make sure you have a pressure switch installed MANUAL reset and a low water cut off and the feeder is working properly.
The hydraulic shock BANGING can rupture a pipe quite easily and this is the reason you want to allow all the water trapped in the radiators to drain PRIOR to having the steam hit these pockets of water. Good luck