Funny you should mention this, I had the exact same thing happen with my 33 yesterday.
Had been driving all weekend, wife took it to the shops came back 30 mins later and it would attempt to turn over but couldn't fully crank.
Checked the voltage and the battery had drained itself. (nothing was left on in the car). Pulled the battery put it on charge for a few hours, put it back in, started no problems at all. Checked for voltage drain and there is none.. just bizarre..