Texture issue - basic but I don't know what happened

Opened my file and texture has gone haywire, what happened please so I don’t do it again.

At a guess, that looks like the model (a scan mesh) had UVs and a texture map in a material and then the geometry was changed in some way making the UVs no longer match. It may be possible to import the original again and use that mesh as a custom mapping object for your edited geometry.