Q: car won't start after sitting for three weeks

In all probability the battery is dead. The reason the horn blared during the jump start attempt is the "restoration of power" is interpreted by the car’s security system computer as an attempt to "steal" the car. That’s just the way the security system is designed. Depending on the age and condition of the battery (the battery must be tested using a load tester), you may need a new battery. If you know the battery is more than 5 years old, that is if it is at about the end of its service life anyway, I would recommend that you request a battery replacement which will likely resolve your issue. If a new battery doesn’t resolve the no start, the mechanic will still be right there to get the issue diagnosed. If, on the other hand, the car’s battery is less than 5 years old, request a no start diagnostic(https://www.yourmechanic.com/services/car-is-not-starting-inspection). You may still need, and end up with, a new battery and the end point of either service will be identical: the mechanic will get your car running, and in all events will verify that your car’s charging system is properly charging your battery. If you have additional concerns, please let us know.