My guesses are; if it's "just" the cambelt then it should spin over pretty freely on the starter motor, albeit possibly with some rattles from the front of the engine. If the starter motor engages but it doesn't turn at all then it could be something major that led to the belt failing. If the rattles you were getting are just the starter solenoid trying but failing to get the starter to do its job then it's electrical and likely to be either a loose main earth connection or, more likely, it's the regulator/rectifier again and the battery has just run itself down. Do you know what voltage is showing on the display?

Regardless of the cause it's a bloody annoying thing to happen, and shouldn't happen.
Good luck in sorting it out, hopefully it is something easily found and fixed.

Calum
M3W Vol1
Vol2