Has this just happened Lesley? 🤔 If it's old damage it's likely to have been the late frosts we had. If it's recent it could be that it dried out in the heatwave we've just had, they like plenty of water. Try scraping a bit of the stem near the bottom. If it's showing green it's still alive, if not unfortunately it's dead.
It could also be the dreaded vine weevil grub 😡 give the stem a gentle pull if it feels loose then I’d tip it out onto a tray and have a good look through the soil, you won’t miss them, although it may just be an idea to do that anyway, good luck.
@pelly @sieveheadsarah hi girls just to let you know that I scraped some of the stem near the bottom of the stem and it is green 😀 so I have given it a good trim and will leave it out of sight and see what occurs in the future. Thanks everyone 🙏