If the drum was off when you stepped on the brake, you more than likely blew the wheel cylinder apart.... that would leak like a stuck pig.
Were the lugs loose? Did it make any noise before it abandoned the rest of the truck? That kind of thing really shouldn't happen........ Not even a really hard bump should rip the wheel off, at least, not without totally trashing the tire and wheel.....
How does everything else look there? Back plate usable? Brake shoes? Etc.?
Lugs are just "pressed" in to the axle. You can use a BFH to knock out what's left, use a spare nut, and thick washer to pull the new ones in. (air tools make this a LOT easier) Fix whatever issues the brakes came up with, put it back together, and you should be ready to roll.