If you can't get above it, cut a piece of plasterboard bigger than the hole, that you can poke through the hole end-on, so it will lie flat on the top of the ceiling. Brush or hoover away any rubble.
I recommend buying a small tub of joint cement, which is a readimixed plaster used for hiding joints and screw heads. It is a bit expensive but you won't need much.
Once you have verified your patch fits, butter it thickly with the cement, poke it through the hole, and weight it down with a piece of string to hold it flat to the ceiling. When it is fully set, snip off the string with scissors, and use the widest filling knife you can buy to spread the cement up against the patch. Use the width of the knife to press against the surrounding ceiling so that the plaster does not project. Use the knife like a razor to shave off any excess. Run a ruler across the ceiling to identify any bulges so you can remove them.
If you get a stainless steel knife it will be easier to clean and last much longer.
It is important not to apply too much plaster that you will have to scrape off later (sanding is even worse). It is easy to scrape or shave off excess while it is soft or cheesy. Plaster tends to crack as it dries so apply too little, and when it is set, finish with the thinnest possible skim to get your final level.
Plaster dust is caustic so wear goggles a mask, and a shower cap, and have your assistant hold a hoover hose to collect it. Plaster dust is damaging so I recommend buying a builders canister vac, preferably a wet and dry one that you can use for future DIY work.
I use the Gyproc one with the yellow lid but here are some other examples
www.wickes.co.uk/Products/Building-Materials/Plaster+Plasterboard/Jointing-Compounds/c/1005002?q=%3Aprice-asc%3AType%3AJointing%2Bcompounds&text=&Jointing+compounds=Jointing+compounds#