If your hill assist light is on, there are a few things that could be going on. First, check to see if your handbrake is engaged. If it is, disengage it and see if the light goes off. If the light stays on, there are a few other things that could be happening.
One possibility is that there is something blocking one of the sensors that tell the car whether or not it's on a hill. This could be dirt, snow, or ice build-up. Another possibility is that there is an issue with the car's braking system. If you have recently had your brakes serviced or replaced, this could be the cause of the problem.
If neither of these solutions seem to solve the problem, it's best to take your car to a mechanic or dealership to have them take a look at it.