Help, my speakers are blown!

If you've worked with, hired, or used sound systems for long enough, you'll come across this scenario.  In my case, two powered subs came back from a PA hire job not working.  A quick test with a multimeter across the terminals of the drivers and a smell of burning confirmed the worst... basically the voice coils had melted.
This is about the most common reason for a speaker to blow.  In my case, the unsupervised DJ had ran the system into the clip lights all evening.  Even with limiters, this can cause square waves to pass through to your speakers, which is bad for both the voice coils and the speaker cones themselves.
The only remedy in the case of a melted voice coil is to replace the driver.  The cost could be from £20 to £400 depending on the speakers you have.  Of course, more expensive equipment is less likely to sustain this kind of damage, but it can still happen, and it's even more expensive when it does!

Driver from a Yamaha MSR800W sub-woofer

Blown voice coil removed from the driver