My response as a PPL and electronics engineer, NOT certified aircraft engineer / technician.....
The warning light means that the starter motor is still spinning. This could be because it has not disengaged from the engine, or is still being supplied with power (e.g. dodgy switch). In the former case, it implies that the engine is spinning the electric motor rather than vice versa. This will burn out the motor (and any other drive mechanisms in between) fairly quickly and possibly seize the whole thing up. In any case, the engine will not develop full power.
That is not good!
The light should go out within about 2 seconds. Otherwise shutdown the engine. Do not wait to see what develops and certainly do not fly or even taxi.
As to what to do next, well, I'm not an aircraft owner so I don't know.
TG