Does anyone know why the light for the airbag on the dash, is staying on for on my 306?

the light for the airbag has just started to stay on , ive looked in the handbook but it just says "if light flashes for 5 mins then stays on see a peugeot dealer"

what can go wrong with the airbag? will the light staying on affect the MOT ? as its due next week

cheers