Just got in the car, went to put the window down to hear a horrible pretty loud crunching noise and now the window won't budge, it's loose to move by hand and the motor sounds like it's working
So what has broken?
Your regulator has broken. They're about £130 from BMW. I changed both of mine when they snapped (the crunching sound you can hear is the metal cable that's now all frayed). It's a tricky job but not beyond a competent DIYer.
Regulator! Mine went a few windows. Had it forced back up to the top, then BMW fixed it under warranty. Can imagine the labour might be a bit more than you'd expect as it sounded a bit time consuming.