I have a 2002 PT Cruiser. Last week, my two year old battery died while sitting out in my work's parking lot. I didn't leave any lights on or anything. I tried jump starting, but it wouldn't budge. I ended up buying a brand new battery and it worked for a total of 3.5 days. My car is now dead again and won't jump start. I have no idea what the cause of this could be. All interior lights and functions still come on, but car won't start. Any help?

 

Maybe I should also mention that it doesn't crank at all when I try to start it

 

Not sure where you're located, but being an '02 model year....
Check the trans shift cable connection to the transmission gear selector, A worn bushing may not let the gear selector sensor read that the car is fully in park (many folk will also call this the neutral safety switch) It is common for the shift cable end to wear(it's plastic) and not allow full positioning at the trans, even though the inside shifter shows proper.
Next would be to check the small wire connection on the starter solenoid for good, CLEAN(no corrosion), tight fit. Usually a female spade metal connector encased in plastic, that the metal connector goes "wonky".
These 2 suggestions should be done, but may not be the actual problem YOU are having. GOOD LUCK!

 

If you have fog lamps, there is a known issue in the headlamp stalk that allows the foglamps to drain the battery. I believe pulling fuse 8, driving/foglamps in the in-cabin fuse cavity will prevent this issue from draining the battery.