Phone bootloaders are unaccessible by common applications. A bootloader locks during a bad flash using an inappropriate firmware or based on the incompatibility between flashing applications or just the flash operation itself. If you performed such operations and something failed during the upgrade or reflash, then a service technician is the best solution available to you. Another possibility is to use HTC support application which is used for Firmware upgrade and maintenance operations. Connect the phone to the computer with the USB cable which is provided with your phone and choose Emergency recovery from the main window of the software. Then perform one if it's available and if this process fails, you should send your phone to a warranty service.
