This may not apply to your particular spa bath, but other manufacturers use a power button that, when pressed, pumps a little bit of air to a remote electronic switch via a thin air hose. These can fail if the air hose splits or becomes detatched from either end, or the button itself no longer forms an air seal. If you can take the side panel off, check to see if your power button is this type and if it has the problems described above. Can be a simple repair if you're lucky and would be within the realms of a plumber to fix if you can't do yourself.
If your bath has electronic control buttons, you'd need to find a specialist spa bath engineer rather than a simple plumber or electician.
Rupert, January 2020
