Are you familiar with ADB ? All you need to to boot your phone into Download mode, right ?
If phone turns on and is recognized as a USB device, you are still lucky. Use ADB and command the phone to boot in download mode. The ADB command (if I remember correctly) will be "ADB REBOOT DOWNLOAD"
You can find more info on ADB and its documentation on the web.
Let's know if it helps. Good Luck