Firstly, do you know what smart you own? Check
A non working horn is usually very easy to sort. Follow these in the order shown until it works.
600cc 450 with
700cc 450 with
999cc 451 with
Forfour all variants -
Pull the fuse out and visually check it, if you have a new replacement, fit it anyway.
The steering wheel has 3 contacts underneath the part you press to sound the horn.
Try pressing all over the horn press to make sure it's not a poor contact.