No sound from tweeters - is it the tweeters or crossover at fault?

switch the tweeters between the speakers - still no sound then it's the crossover. If you get sound using the other tweeter, then it's the original tweeter that is bad. (or use a cheapo tweeter from Fry's or something if you don't want to risk your other working tweeter)

I am voting with others hear that it's the tweeter. a crossover failing and killing output to a tweeter is unlikely.

I recently had to fo this with my Revels and it was the original tweeter that was bad.

Can't imagine there being a crossover problem, however it is possible.  It is most likely the tweeter.  I believe that the CM5s2 has a rear plate,  remove that plate and find the lead that goes to the tweeter,  Measure across the leads with the ohm rating on a meter, being an 8 ohm tweeter,  you get a dc resistance measurement of around 6 to 7 ohms or so. Tweeter is good.   If you don't get a reading, they you have a broken voice coil,  if you get a shorted reading then you coil has been burnt.  In either case, you would need to replace your diaphragm.   Ask B&W if they sell a replacement diaphragm for the tweeter... Also,  If I am correct, B&W uses electrolytic caps on this series,  while you are repairing them, replace them with a decent polypropylene (even a cheap polyprop) and you will get a clear improvement on sound. 

Do your speakers have a fuse? One speaker I owned had a fuse to protest the tweeters only - the mids and woofers played without a problem - tweeters were dead.

Changed the fuse and all was good again

Regards - Steve

use conventional multi-meter to test out speaker crossover components one after another disconnected (one lead enough). not too many and no effort. start with higher power resistors.