I can pretty much guarantee that the tweeter has blown and needs replacing. Very common problem .
Can be confirmed by measuring it's resistance with a DMM. Should be low ohms (about 7ohms) but yours will be open or closed circuit meaning it is defective.
Should not be that expensive to fix but you will need to take it to a B&O service center. In the UK the tweeter costs around £25 plus the cost of fitting.