I need help Bad! My 2000 Plymouth Neon wouldn't start, I realized my battery was worn so I bought a new battery. When trying to connect the new battery at night I connected the wires to the wrong posts! I got a spark, but didn't stop there, then I saw fire & smoke! I quickly removed the wires & corrected the connection. When it was properly connected the car started. The next day the battery indicator on the instrument cluster came on. My friend disconnected the battery ground wire while the car was running, this caused the engine to stop. He said that the alternator was bad. I bought a new alternator, had my friend install it & now the same problem. It seems that the alternator is not charging the new battery. Do you think I ruined something electrical?

check the fuses first

Check for power at the charge line at the alternator. That is the large single wire at the alt. Voltage goes from the alt through a fusible link to the starter and battery. If you have no votage at the alt you have fried the fusible link.

Check grounds also make sure none fried and burn loose while the [polarity was reversed. A good visual check will go a long way.
good luck

Thank you all, it was a fusilble link in the alternator power wire!