Airbag icon lighting up on Opel Astra dash, what does this mean?

If the airbag light is on the dash then none of the airbags will work. Take it to the dealer to get them to diagnose where the problem is. It could be the airbag itself, clockspring, module, wiring problem, fuse etc. It could be anything so there's no point speculating. Depending on what the problem is as to what it costs. Remember though, that they're there for safety & if you didn't get the problem fixed you might regret it in an accident. Also, depending on where you are, it is enough to make the car unroadworthy.