The operating system for my ipod Nano 7th generation was deleted - how do I restore it?

... just a blank lit screen with nothing but do not disconnect

Well, that's not "nothing".

My advice is let the iPod run out of power. Once it has, plug in to a power source and wait to see what happens.

It's possible that the iPod is simply stuck doing something and therefore unable to respond. By letting it drain the battery completely, the iPod will stop doing whatever it is doing. It will drain the battery if there is a message on the screen, even the do not disconnect one. Once it gets enough power to start again, it should start from scratch.

So -

• unplug all the cables from the iPod, no power outlet, no computer connection and no docking station
• leave the iPod alone for several hours, say while you are out at work or school, or overnight
• once you notice that the screen is blank, leave it for a further hour (to make sure the battery has run down far enough)
• now - and only now, plug the iPod into a power supply, preferably a wall charger or docking station
• after about thirty minutes, the iPod may respond. However, leave it to fully charge before trying to do anything.

What happens?

Try this Apple article: httphttps://support.apple.com/en-gb/HT204095 If iTunes doesn't recognise your iPod. Hopefully, it will help.

In theory, it's not possible to delete the iPod's software. What symptoms are you experiencing, what is the iPod doing, or not doing?