I had the same with my 75 this weekend
Left the car for a few days without starting and then it won't turn over. I'd check that the battery holds a charge first - charge it up and then see if you can turn the lights or wipers on after leaving it for a while. If it still doesn't hold a charge you'll almost certainly need a new battery. If it does, and the car won't turn over it's probably an alternator/solenoid/starter motor/wiring problem.
anyways, hope you get it sorted!