There is a lot on this forum about this. Often it is as a result of moving a seat with the ignition on and is related to the under seat connector. This happened to me about 3 months ago when I lent my car to my brother, and I have just done it again myself when I moved the passenger seat to fix something and did not realise the ignition was on.
I have alfadiag on a laptop and use a cable to get into the ECU and switch the light off. It's not 100% straightforward (you have to bridge some pins to get to the airbag ecu). In fact I am going to do it right now.
All the best