You can't replace the head of a screw. A screw is one solid piece of metal, including the head. The head of that screw has sheared off, either because of over-tightening or stress.

You need to remove the handle, extract the remnants of the screw with a pair of pliers and replace it with a new screw. If you can't get a grip on it with pliers, you'll need to use a screw extractor tool. Once it's out, you'll need to replace it with a new screw.

Mostly >>3066 but before you go out and buy a screw extractor tool, see if you can just rotate the disc plate a few degrees and re-screw it with one fresh screw, re-using the two that aren't broken.

>>3066
>>3067
No no you're misunderstanding. The screw itself is fine and it has a head. It is entering from the opposite side of the door. You're looking at the bottom of the screw. The hole goes all the way through the door. I just need something to screw onto the bottom to secure it, otherwise it will fall out.

>>3068
So it's a bolt rather than a screw? In that case you need a nut for it.

Holy shit this has to be a troll.

>>3068

Assuming this isn't some weird troll, then what I think you're describing is a barrel nut.

>>3070
>>3071
Why do you think it's a troll? Am I asking a weird or stupid question? I'm just looking for an equivalent to the thing that I lost. I drew a picture of it. Maybe it came with the door handle I dunno. Not sure if a barrel nut is the right thing.

>>3072

That's a nut m8. It's round and it's got a slot in it, so you want a slotted round nut. If it's a metric thread, then you just need to measure the outside diameter of the screw around the largest part of the thread to find the right size. If it measures 4mm you need an M4 nut, if it measures 5mm you need an M5 etc. If it isn't a round number of millimetres, you've probably been cursed by the Imperial Fasteners Fairy and I'd suggest taking the bolt into a decent hardware shop.

https://www.ebay.co.uk/itm/114307043178

>>3073
Fair enough I suppose, thank you. I'm just worried about finding the right size for the hole because nuts are generally quite thick, whereas the thing I lost was very thin and a bit tapered and probably came with the door handle to fit the hole perfectly flat.

>>3072
I think perhaps you're describing an insert nut. You screw it in with an allen key, and then it takes a bolt through the center.

https://www.screwfix.com/p/insert-nuts-type-d-m6-x-13mm-50-pack/59937

But I would also go with the other suggestion in the thread, to simply rotate the entire thing by a few degrees and just use normal screws. Captive insert nuts like this are usually used to fix bits of furniture together, that might need to come undone later - something like a large worktop, desk or table.

>>3074
For 5 seconds of googling it seems it might be called a sleeve.

Rummaging around on Amazon, and found these - door handle screws. I'm sure it's far too late for OP, but if anyone hits the same problem, these exist. Snip them to the right length without buggering the threads, which is a nice touch.

>>3118
No I'm still here and still haven't fixed it - that's great, thanks!