user35118: I have a V2 Adafruit Gemma from the Gemma starter pack, and I'm running Windows 10 Home (version 10.0.14393), on a 64-bit laptop. Following the  Introducing Gemma guide I've installed the Adafruit drivers and I'm now trying to get it to connect via USB. 
After connecting, the board shows the green power LED, but not the pulsing red LED, and Windows shows the 'USB device not recognised' message.
Tried so far

I'm definitely using USB 2 ports, and I have tried it connected directly to the PC and via a USB hub.
I'm pretty sure the cable is working correctly (It works for exchanging data on a phone, and I've tried a few different ones).
Under Device Manager, the board shows as 'USB Device (Device Descriptor Request Failed)'.
'Update driver software' > 'Search automatically...' reports that 'The best driver software for your device is already installed'.
If I do 'Update driver software' > 'Browse my computer...' and point it to the manual Adafruit driver files (available on the link to the guide), I get the same message.

Can anyone suggest anything to try? Do I need to (re)install the bootloader itself, and if so, can you point me in the direction of how I do this? 
Update
As mentioned in the comments I have posted on Adafruit's own support forum but got absolutely nothing yet. I have also tried it on my work PC (Windows 10 Pro, 64-bit) and got exactly the same behaviour.
I'm keen to get this sorted so bounty added for anyone who can at least come up with some suggestions of what to try!

user35409: I think, you've pretty much done everything. If you could gain access to another board, and try that one you could figure out more. Otherwise your symptoms remind me of a something called an ESD (Electrostatic Discharge) caused malfunction. This basically means one of the system essential chips on the board have been fried. It is very easy to ESD a chip without protection, just walk on a carpet or put on a sweater, and than touch the chip. Chances are you were electrostaticly charged with thousands of volts. The chip does not stand a chance. To prevent this from happening, touch something made out of metal before touching the board.More on ESD: Electrostatic discharge

35118: <p>I have a V2 Adafruit Gemma from the <a href="https://www.adafruit.com/product/1657" rel="nofollow noreferrer">Gemma starter pack</a>, and I'm running Windows 10 Home (version 10.0.14393), on a 64-bit laptop. Following the <a href="https://learn.adafruit.com/introducing-gemma/windows-setup" rel="nofollow noreferrer"> Introducing Gemma guide</a> I've installed the Adafruit drivers and I'm now trying to get it to connect via USB. </p>

<p>After connecting, the board shows the green power LED, but not the pulsing red LED, and Windows shows the 'USB device not recognised' message.</p>

<p><strong>Tried so far</strong></p>

<ul>
<li>I'm definitely using USB 2 ports, and I have tried it connected directly to the PC and via a USB hub.</li>
<li>I'm pretty sure the cable is working correctly (It works for exchanging data on a phone, and I've tried a few different ones).</li>
<li>Under Device Manager, the board shows as 'USB Device (Device Descriptor Request Failed)'.</li>
<li>'Update driver software' > 'Search automatically...' reports that 'The best driver software for your device is already installed'.</li>
<li>If I do 'Update driver software' > 'Browse my computer...' and point it to the manual Adafruit driver files (available on the link to the <a href="https://learn.adafruit.com/introducing-gemma/windows-setup" rel="nofollow noreferrer">guide</a>), I get the same message.</li>
</ul>

<p>Can anyone suggest anything to try? Do I need to (re)install the bootloader itself, and if so, can you point me in the direction of how I do this? </p>

<p><strong>Update</strong></p>

<p>As mentioned in the comments I have posted on Adafruit's own support forum but got absolutely nothing yet. I have also tried it on my work PC (Windows 10 Pro, 64-bit) and got exactly the same behaviour.</p>

<p>I'm keen to get this sorted so bounty added for anyone who can at least come up with some suggestions of what to try!</p>
 35409: <p>I think, you've pretty much done everything. If you could gain access to another board, and try that one you could figure out more. Otherwise your symptoms remind me of a something called an ESD (Electrostatic Discharge) caused malfunction. This basically means one of the system essential chips on the board have been fried. It is very easy to ESD a chip without protection, just walk on a carpet or put on a sweater, and than touch the chip. Chances are you were electrostaticly charged with thousands of volts. The chip does not stand a chance. To prevent this from happening, touch something made out of metal before touching the board.<br />More on ESD: <a href="https://en.wikipedia.org/wiki/Electrostatic_discharge" rel="nofollow noreferrer">Electrostatic discharge</a></p>
