Is it like this?

string end

https://1.bp.blogspot.com/_xWjoeLNcmQo/TMBynP8FjiI/AAAAAAAAFgQ/IvZV14cdTRM/s320/guitar+string+ball+end.jpg

If so this is a standard steel string guitar string. If not can you supply an image? The simplest guitar nut repair is to fill with bone dust and super glue. If the surface is wood you might want to try wood dust and glue to raise the surface or glue in a small piece of wood, the harder the better. Again a picture would help.

The blue clicky isn't working but the URL is OK