Major bummer. Your car got too hot, too much light. Over 85 degrees F the plants 'turn off'...above 90 and they start dying. Your basil is toast, literally.
Take those leaves, lay them out on a sheet of parchment and allow them to finish drying. Put the leaves in a jar, open the jar once a day for a week and then you have freshly dried yummy basil.
When you purchase a plant take note of its environment. If it is inside, it has to be inside, if it is out side in the sun, it has to be outside in the sun, if it has been in a greenhouse, you need to get as close to that same environment until you are able to acclimate that plant to a new environment.
Heat will kill plants, changing the environment too quickly before helping the plant to grow thicker or thinner epidermis (skin) to handle full sun or full shade...will easily kill plants as you have found.
This is a bummer but please know it takes mistakes to be able to be successful as a gardener and this site can greatly help diminish mistakes, we all had to make to become professionals.
I hope this does not deter you from trying to grow plants! This pot of water beneath your plant is not at all a good idea, not the reason this plant died but it will kill future plants, unless they are water lilies. Next plant please holler at us first and we can walk you through the steps and make sure you are successful the next time! You still have a year's worth of dried basil that will be far superior to store bought dried basil! (Try Thai Basil next time...)
