Try swapping lights from the blown section into the working section of the tree. If the previously working section starts working again then you know the bulb was OK. Continue swapping out each bulb in the blown section until you have found the one that doesn't work or the entire string has been tested. If you make it all the way through the blown section and every light tested in the working section are OK then it is a wiring issue that you will not be able to fix easily.