When the key is in the ON position, check that you have 12v applied to the purple wire terminal of the horn and that the black ground wire is indeed grounded.

Find the heat sensor wire (usually TAN) that protrudes out of the cylinder head and ground it to the powerhead somewhere. This should make the horn sound constantly. Does it?

If the horn sounds as it should, the horn is wired properly but the self test portion has failed. You can rig your own test setup simply by having a wire added to the TAN wire at the horn and have it attached to a self centering toggle switch terminal with the other terminal of the toggle switch grounded. Pushing the self centering toggle switch off center will cause the horn to sound.

If it fails to sound when you ground the heat sensor TAN wire at the cylinder head and you're sure that 12v is applied to it, ground the TAN slide on terminal of the horn. If it still doesn't sound, the horn has failed.
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If by chance when you finished using the engine last (a month ago), you ran the engine out of gas, that would allow the oil side of the VRO to continue pumping... and that loads the carburetor float chamber up with oil.

Remove the float chamber drain screw/bolt to see if this took place (full of oil) and also check to see that the proper mixture is eventually entering the float chamber.
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Possible that the carburetor is somewhat fouled after sitting but unlikely if just for one month.