Hyundai Repair Questions? Ask a Mechanic for Answers ASAP
If the alarm was active the engine would not start, the system was likely damaged when it was stolen. In order to determine what is causing this you are going to need to start with a scan of the system to see what fault codes are setting.
If it does fine start then its not likely an alarm problem, if the alarm was active it should not start. The problem with the radio would not affect the running. It may have been damaged when it was stolen.
If the horn and hazard light are active then the alarm is active and in order to know why it will not disarm you are going to need to have the system scanned and the fault code retrieved to know where to even start to repair it. If the remote is not working that is likely why you can not disarm it.