Yesterday, the only vector processing tool I had was Open Street Map. After checking here, I re-installed QGIS and I had all the processing tools.
This morning, I have been using Geometry tools. When I deleted the layer I had created using Export/Add geometry columns, several processing tools, including Geometry tools, had disappeared.
I closed down QGIS and re-opened it and they were all back again.
Has anyone any idea what is going on please. It is annoying to have to close down and restart to be able to use a particular tool.