Plasterboard can easily become damaged often due to impacts or perhaps where a electrical switch or socket has been removed, this leaves a unsightly hole which will need filling but because there is a void behind the plasterboard it is not simply a case of filling the hole with filler as it will drop straight through and into the void. This method is suitable for small impact damage, for larger damage you will need to cut the plasterboard back to the studs.
Start by cutting around the damaged area and make it square and neat-
You can cut the plasterboard by using a dry wall saw as shown here Cutting plasterboard
Then cut a piece of scrap plasterboard or 1/2 inch thick piece of wood and cut it slightly narrower than the hole that you have just made but make sure it is at least 1" longer at each end-
Now make a hole in the centre of the plasterboard or wood that you have cut and thread a length of string through the hole and tie it around a nail or something similar to prevent the knot from pulling through the hole. Now apply some adhesive to each end of the plasterboard, dry wall adhesive is best but any instant grab adhesive should work.
Thread the piece of plasterboard or wood through the hole and keep hold of the string firmly, gently pull the string towards you so that the adhesive spreads and grabs, the adhesive should not slip, providing it is good quality! If it starts to slip use a couple of dry wall screws to hold it in place until the adhesive has set.
When the adhesive has fully set you can cut the string and fill the hole either with a proprietary filler or some bonding/browning plaster and a final skim of finishing plaster, you can always sand it with a sander if you cannot successfully plaster.