If you troubleshot the simple stuff and it's not reading at all, my guess is the USB port is failed. If it didn't just happen to me with the exact symptoms I'd suggest softer fixes.

I used a card reader [I also keep an extra storage card for this kind of thing to speed up the process]. You'll need ot make sure there is NO encryption on the card and that it's formated properly or flashing from the card won't work. Use the card reader to load the nbh to the SD...pop it in your device and put the device into bootloader. I actually got pretty slick at flashing from the card and now kinda prefer the method. Def get a card reader though...even if it turns out not ot be a hardware fail, the card reader has saved me countless times.

Here's everything: http://forum.ppcgeeks.com/showthread.php?t=61333

..but yea, as root said, you'll want to exhaust the softer fixes first to rule out anything simple. If you're not getting anywhere, lock it up and bring it in. Sprint replaced mine no hassle.

EDIT: I would highly recommend all flasher folks pick up a cheapo card reader and an extra storage card that you keep loaded up with a Stock ROM and a relocker [maybe even an unlocker and your fav NRG ROM build ]. In case of a major issue like this, you won't be up the proverbial creek...