There is a fairly common problem on the Roadster, where the horn will stop working.

The first point of call will be to check the fuse (number 31), but if this is okay, then it is most likely due to a poor connection on the contacts from the horn.

Firstly, remove the bonnet bin using a 2p coin (or similar!)

Looking into the bay, the horn is located here:

Start by pulling the connectors off, the 'lower' connector is the negative, and will typically be more tough to remove.

Just use some fine grit emery paper (or similar) to wipe the surface of the connector on the horn.

Replace the connectors and test.

It is as easy as that!!