Not clear what you mean by collapsed.
If the electrode was flattened out the piston may have gotten loose and "crunched" it. Maybe you only lost a rod bearing or a wrist pin went sour, all the connecting rod bolts look good? Check them by hand to make sure nothing stretched or broke.
Perform a
slow turnover by hand with all the plugs pulled making sure each piston is following its correct time and path. If that works out it seems a slow disassembly is in order until you can locate the sore spot.
There are way too many variables to do any real analysis without more specs on your engine, give us more details of the motors behavior when it went boom, before and after, did you check
oil temp.
oil pressure, engine temp? Maybe post some pictures of the plug and any other areas you suspect.