If system has been emptied or nearly emptied, it may be necessary to put some rust inhibitor in to the radiator system. Start bleeding closest to boiler and work your way further away, with a combination boiler keep topping it up via the filling loop, to maintain a pressure above 1 bar whilst bleeding.
With a conventional vented boiler (hot water tank in cupboard somewhere and feeder tank in loft), if the system has been emptied, or nearly empty, it may be necessary to bleed the circulation pump, which is normally fitted close by the hot water tank, but may be fitted elsewhere in some situations. To bleed a pump, release the screw cover in the middle of the pump and use a rag to catch any dribble of water. Turn the boiler to its lowest heat setting and select central heating on. Once all air is out, you can then turn the heat setting on boiler to your normal level.
If you are having problems, you can post in job section to get a plumber to fix.
Hope this helps