If your computer cannot recognize your BlackBerry on the second screen of the Loader.exe file, remove the battery and plug the phone into the computer. As soon as you see the device PIN or PIN: UNKNOWN pop up, click Next. If you miss it, unplug the device and plug it back in (note: when you're doing this, you should get a red LED as the phone attempts to turn on). Once you're able to catch the phone in the reboot cycle, put the battery back in and proceed with the OS reinstall.
If you get no red LED at all when you try plugging your phone in to your computer, something has likely happened to your phone's hardware. If this is the case, the only option would be to take the phone into your carrier for repair, especially if it's still under warranty.
I hope this info helps!
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