It's a good question, because i think that seems like maybe the most likely instance where a nuke might be used - they're best placed to "get away with it." I think the important things to note are that:

1. russia doesn't want to spoil the land/living space in the short term, needing to make back cost of war.

2. ukrainian controlled territory, though far from moscow, is bordering russia (and crimea) - russian towns near the border will be affected, and regardless of that, during the chernobyl crisis all countries in europe, maybe even in the world, suffered from the fallout effects. I believe nuclear powers aren't willing to take this gamble- as large countries there's a good chance one of their territories will be affected, as will other countries, who then sour on them.

3. America is ~~run by~~ lead by you know right now, but under any different president, russia should expect retaliation, and they still might expect retaliation from NATO. It's never worth nuking somewhere just to get nuked yourself.

As for what actually happens, i would honestly hope nobody nukes russia back but rather that it gets invaded by NATO in retaliation, like cattywampas said. But reality is often dissapointing.

My bet is that Russia probably gets invaded but nobody else uses a nuke.

At some point the idea was that NATO would retaliate using conventional weapons and quickly "destroy" Russia with precise long-range strikes targeted at critical military infrastructure. But with Trump you know it won't happen. Besides, we saw with Russia in Ukraine and with United States in Iran that this doesn't work and there is no quick/easy way to win a war like that. So to answer your question: probably nothing.