This is how my day went
 

Drove around most of the afternoon in my '02 S just running errands n stopping at different places on the beach to relax for a short while..every time I would climb back in the Boxster n it cranked right up..got back to the condo n parked in the garage n cut the car off. As I was getting out I decided to go ahead n put the top up..so I climbed back in inserted the key n turned the switch....nothing..would not turn over. Got my jump box from my truck n connected it..would not turn over. Granted my jump box was not fully charged n I haven't had time to check everything out but that was my day..I've had the S for 10 months n the battery was in it when I bought it so maybe time was up on that battery but it was so sudden..cranked all day n then it was dead? Hard to believe with no warning at all..has this happened to anybody else? I'll be looking closer tomorrow but until then your comments will be appreciated....EZY

Start with electrical. Check your battery voltage. Could be a crankshaft position sensor failing. You can ohm that out. Could be a relay. Could be a fuse. Could be your immobilizer. Could be just your ignition switch.

If it was the CPS the car would turn over.

Any click at all? If none, key switch? My car cranked slower and slower with a batt issue.

Happened to anyone else? Yes! If your battery decides to crap out completely, your jump box will not work if connected in parallel. It may work if your dead battery is disconnected and your fully charged jump box is connected to your battery leads.

In January of 1999 I leased a new Z3. In the spring of 2000, I was picking up one of my boys after school and upon getting back in the car, the battery would not even give a 'click'. I couldn't believe I had a bad battery after just a little of over a year. We caught a ride home and I took the battery out of my 84 Alfa and installed it in the Z3 and the car started right up. As the BMW was under warranty, BMW insisted on having my car flat bedded an hour away to replace the battery as opposed to simply reimbursing my for buying a battery locally...

put it on the battery charger over night. Also check your battery connections, make sure they are clean and tight. then check the battery cable connection to ground and check the positive battery cable connector to the starter.

In my experience, batteries will work fine right up until the day that they don't. No slow cranking or other symptoms - they work fine one day and then don't work at all. Have the battery tested at a local auto parts store.