It's possible that it's the driver itself. Do you have a multimeter? If you measure the DC resistance of the driver, it should give you a reading. If it doesn't, then the voice coil is an open circuit, and is blown.
If it is the tweeter, you can probably source a suitably close replacement from a speaker distributor like Madisound. My experience with B&O speakers is that they usually use SEAS or Vifa tweeters. I've heard they use Phillips drivers as well sometimes. If they are one of those brands, I bet you could find a very close replacement from their new lines. The new SEAS tweeters, like the 27TDF are great! If you pull the tweeter out of your speaker and the number on the driver starts with an "H", it's a SEAS driver.
If the driver measures out, then you should look at the tweeter section of the crossover filter. Some component there is no longer functioning - a resistor or capacitor.
Hope that helps.
Jon