Selectively prune the longest branches--no more than 1/3 of the branches--back nearly to the base, and feed with a half strength dose of a soluable rose food. After a few weeks new sprouts will start filling in the bare spot. If it has gotten truly ugly, it may be better to cut the whole plant back to stubs, but that will delay bloom a couple of months.Also make sure it is getting plenty of direct sunlight.