The battery in your car will drain a little bit every day if you don't put a battery maintainer on it. After sitting drained / empty for several months, the battery is probably completely dead and will probably not come back to life again. I would replace the battery, particularly if it's the original one in the car. I have an article on this right here, as well as the steps required to install a battery maintainer:
http://www.pelicanparts.com/techarticles/Boxster_Tech/81-ELEC-Battery/81-ELEC-Battery.htm
- Wayne