If it has simply popped loose and the solder is still there on the horn and on the post, try this first.

I use floral arrangement wire. you can get it at hobby lobby or a craft shop or maybe even the dollar store.

1 a touch of soldering flux on both surfaces popped loose.

2 wire down the post in position

3 use a heat gun to heat up the area. Helps save the lacquer also.

Have a torch ready. Look for a boiling in the flux.

4 immediately and lightly, touch the post solder area with the flame to quickly melt the solder again.

Unless the solder has been removed, you simply need to get is liquified for a brief second to "pop back" into soldered on position.

If you need to use more solder, use a 40% real silver solder. It needs the strength.

Good Luck