My sister in law tried to install a new thermostat and has somehow messed something up. I went over and her Trane air handler will not run. I found that the glass bus fuse beside the transformer was blown. I replaced the fuse and I still get nothing except the hum from the transformer. I disconnected all the low voltage wire from the air handler thinking maybe there was a short somewhere and I still just get the hum. Any ideas what I should do next? I wouldn't think the transformer would be humming with all the low voltage wires disconnected from it.