Agree with Fluff... at this point it'd be near impossible to tell what happened for sure. I guess I'll rattle off a few questions to maybe narrow down the possiblities...
1. What order did things die off? Did they go over the course of a couple weeks, or are we talking instant death over one night?
2. What are your tank specs - how much live rock, how deep of sand, what type of equipment do you have running (filters, skimmer, heaters, lighting, etc)
3. Do you test your water weekly or often? What were the "normal" water parameters (ammonia, nitrite, nitrate, pH, salinity, temp, alkalinity) before the crash?
4. What type of water do you use to make up your saltwater? (tap, ro
, store bought, etc)
I really don't think the bristle worms are/were your problem. They can't really cause a tank to crash.