How did you try to unroot? It's weird if you don't have working fastboot or flashmode, and without those, you're pretty much screwed. Those two should work even if you screw up big-time, like flashing a wrong kernel, but you must have done something weird if they stopped working?
And let me get this straight, you are pressing and HOLDING the volume buttons when you plug it to your computer, right? Make sure that the device is completely off by pressing and holding POWER + VOL UP. It'll vibrate a couple of times. Note that you're not supposed to be connected to the computer, since it'll then just boot again.
Then press and HOLD VOL UP for fastboot (blue LED), OR VOL DOWN for flashmode (green LED flashing shortly)
You probably already know these steps, but we have to make it absolutely certain that it's a hardware issue, and not an issue with the thing you do