I had this problem just few weeks ago. The code-Alarm system gets re-set when you disconnect and reconnect the battery.
So the options you have are -
1. DeActivate the Code-Alarm System.
This is is what I cud do after trying for 20-30 minutes.
2. Disconnect the Code-Alarm System.
Start your car and then re-connect.
If you disconnect - you may have to find installation manual before you do that. Here is such an example:
http://www.crutchfield.com/learningcente鈥?/a>
Good Luck.
-N