Fill edit and keyboard shortcut have stopped working.

Hi, can anyone tell me why the Fill command has stopped working please? I'm using 2019 and all was going well thn suddenly the usual shortcut (Shift+F5) will not result in filling either a selection or a new layer. All the route is there, I can select a colour, but nothing appears on the layer. Same happens when I go to Edit/Fill. I have the correct layer selected, go through the route of selecting a colour, but nothing appears. Using the Alt or Option key and picking a colour with the Brush tool eye dropper also fails to change the foreground colour - whether that is connected to the Fill issue I have no idea but it was coincidental. Any help much appreciated.