Ctrl+A is a standard Windows shortcut used on all versions of Windows. It works on all Windows built-in programs.
If your Windows standard shortcuts don't work, it's because someone managed to change them, or because your System is corrupt.
To see whether other programs have been changed, Open Wordpad, type a few lines, and see whether Crtl+A selects all or not.
If it's only Notepad that is corrupt, I would suggest installing Notepad++ and using it instead of Notepad.
Glad we could help.
If your Windows standard shortcuts don't work, it's because someone managed to change them, or because your System is corrupt.
To see whether other programs have been changed, Open Wordpad, type a few lines, and see whether Crtl+A selects all or not.
If it's only Notepad that is corrupt, I would suggest installing Notepad++ and using it instead of Notepad.
Glad we could help.