How to reset an unresponsive iPod Classic in iTunes or Apple Devices app?

Hello,
I wanted to put new music on my iPod today. There was a problem, and I had to unplug it.
Since then, iTunes hasn't shown it. The display says it's connected, but I can't access it.
Several restarts (of the PC and iPod) didn't help. Uninstalling and reinstalling iTunes also didn't work.
Then I tried the Apple Devices app. Here, too, it shows the iPod, but it can't get past the loading screen.
Now I'd like to completely reset it to factory settings, but I can't find a way to do that.
Can you help me?

[Re-Titled by Moderator]