You changed your oil "to" synthetic? Hopefully it already had synthetic.

If the light is on, there is no reset for it and if your level checks good, then the sensor may have gone out - it happens. My light has been on since I've had the car (almost 2 years) and nothing bad has happened. Sometimes it will go out for a while and come back on. Not sure why, but I'm assuming the sensor is bad. Since I'm not seeing any adverse effects from it, I see no reason to spend $100 to have a shop tell me my sensor is bad. I keep an eye on my oil level with the dip stick and make sure I change it when it needs it.