Oof, that's the worst kind of surprise. Sounds like you bought the "premium" brand of instant potting soil. For heavy stuff, I've never had Oasis brick foam fail on me.
Check if you were using the green foam for fresh flowers or the brown stuff for dried arrangements. That mix up alone can cause a total collapse. What kind of water weight are we talking, like a big vase full of branches or just a heavy handed bouquet?