I accidentally pressed some unknown key combination, and now I can't use Undo nor Redo in Blender
If you are using English Software (Blender) under Windows based Operating Systems but with an AZERTY keyboard, accidentally hitting Shift+Alt keys toggles keyboard layouts, as explained here.
This apparently "disables" the Undo/Redo operators, when in fact what happens is it changes the keyboard shortcuts
Ofcourse they still work, but now you need to use Ctrl+W to Undo instead of the usuall Ctrl+Z, or use Shift+Alt to toggle back to your usual keyboard layout.
You can also disable the shortcut in the Regional Options of the control panel if you follow the steps described here
Credit goes to 3fingeredfrog for the explanation