Passenger seat belt light stopped working

Hello guys, just bought a week ago a 2006 Toyota Prius with 118k milles and im loving it so far.
Now the thing is, the seat belt light was working perfectly, when a person sat down the light showed up, and when no one was sitting it turned off, but now it simply doesn't work.
When someone is on the seat the light stays off, now my question is? How/What i can start to look to identify this? My concern is the passanger airbag is not working at all or is on all the time, and i dont know what it can be.

Thank you all!