A disabled state shows that a control cannot be used right now. It is like a door that is temporarily locked, so the handle will not open it.
A form's Submit button may be disabled until all required details are filled in. The page should explain what is missing so someone knows how to make the button available.
Make disabled controls look different but still readable. If the reason is not obvious, tell people how to continue instead of leaving them to guess why the control does nothing.