Disconnect wires from tweeter and measure across the driver terminal with an ohmeter. You should get a resistance value of 3-5 ohms. If the meter does not deflect then the driver coil "could"be open. It could also be a poor solder connection somewhere in the crossover, pots, or even at the voice coil connection on the driver. Don't rule anything out.
If $ are not a factor replace or rebuild them as mentioned by others. Good Luck!