The horn is in effect a near-short to begin with. It does not take much for it to become a full short to ground. The horn coil could've shorted out internally, or one of the wires could've broken and shorted to the frame.
You very likely blew the second fuse you put in. If that is the case, do not continue to put fuses in. DO NOT PUT A BIGGER FUSE IN. DO NOT PUT A BIGGER FUSE IN.
You need to figure out what shorted, and where. Step one is disconnecting the positive wire from the horns. Put the same size fuse in then. If that blows, your horns are good but you have a wiring issue. Take it to a shop or break out a multimeter. If the fuse stays good, plug one horn back in. If the fuse blows, that is the faulty horn. If it does not blow, the other horn is faulty;replace it.