Hyundai Repair: My car wont start, crankshaft sensor, hyundai sonata

Question
I have a 1999 Hyundai Sonata about 109,000 miles with a V6 engine. My problems started about a week ago when I could not find my alarm pad so we kept opening the car door and setting the alarm off to find out how to disable the alarm so it could be driven. So when I found my alarm pad a couple of days later my car would not start, we put a new battery in it wrong at first which caused some sparks. When we got it in right my alarm and radio did not work and the light that tells you the door and trunk stayed on even though the trunk and door were closed. My car drove for a day then it started to shut off on me. Took it to the mechanic had the alternator and belts replaced and fuses checked, mechanic still can not get the car to start. Mechanic thinks it is the computer but he is not sure, what do you think it is because the mechanic is stuck he called Hyundai and they don't know. Thank you please help.  

Answer
Hello Nataya,
Honestly this is a tough situation. When you install battery backward - even for a couple seconds- you can damage a lot of different parts in the electrical system. You need to go step by step and fix each problem. First, check all the fuses -inside and outside-. Then check if you have spark at the wires while cranking. If you do not have spark, you might be damaged the crankshaft sensor. Ecm might be damaged due to wrong battery isntallation but before you decide to replace it -it is really expensive- , you need to eliminate all other factors.