bought a used infinity j30, ran okay for a while. Then I INCORRECTLY opened the doors, which set off the alarm, now vehicle wont start

The key should have reset the alarm. If you place the key in the driverside door, lock then unlock the door, make sure all the doors and the front hood and truck hood are closed. Some systems require the negative side of the battery to be taken off inorder to reset the alarm but I found that the alarm was waiting for me when I reconnected the cable. It didn't make any noise but the lights would blink off and on.

i had to go under the dash above the gas pedal and disconnect the alarm harness it stopped doing that i plugged it in about 5 hrs later and it worked like new again