Q: I can't sync my IPod since windows 10 downloaded itself

Check this article: iTunes 11 for Windows: Set up syncing for iPod, iPhone, or iPad

and probably this link will also be useful for you: Re: Ipod Won't sync to Windows 8 PC (LynneGee's answer)

iTunes don't seem to play well with an in place upgrade of Windows.

For general advice see Troubleshooting issues with iTunes for Windows updates.

The steps in the second box are a guide to removing everything related to iTunes and then rebuilding it which is often a good starting point unless the symptoms indicate a more specific approach.

Review the other boxes and the list of support documents further down the page in case one of them applies.

The further information area has direct links to the current and recent builds in case you have problems downloading, need to revert to an older version or want to try the iTunes for Windows (64-bit - for older video cards) release as a workaround for installation or performance issues, or compatibility with third party software.

Your library and device backups should be unaffected by these steps but there are links to backup and recovery advice should it be needed.

tt2

QuickTime is no longer a core component of iTunes. It is occasionally needed to support older video codecs when they're not supported natively by hardware or the operating system. iTunes will issue a warning if it detects content that it cannot play. See Download iTunes for Windows (64-bit - for older video cards) for an example of that warning.

It isn't entirely clear if you're saying that media is missing from the library, or from the device.

See:

• Empty/corrupt iTunes library after upgrade/crash if your library previously held all your media.
• Recover your iTunes library from your iPod or iOS device to grab what is on any device and not in the library or computer.
• Backup your iTunes for Windows library with SyncToy to preserve your library in future.
• Corrupt iPod Classic if media has gone missing from the device and isn't stored elsewhere.

tt2

Did your library hold all your media before the upgrade to Windows 10?

If so, have you looked to see if the media is still inside your \Music\iTunes\iTunes Media\Music folder?

If it is see Empty/corrupt iTunes library after upgrade/crash.

For advice on getting the device to show up in iTunes see If iTunes doesn't recognize your iPhone, iPad, or iPod - Apple Support or Re: How to connect my device to iTunes?

If this is a manually managed device, and you didn't previously keep copies of your media in your main library then see Recover media from an iPod, followed by Backup your iTunes for Windows library with SyncToy to make sure that your library is secured going forwards.

tt2