I’ve not seen a tractor engine without freeze plugs, so that’s some built-in defense. As Chickn mentioned, freeze plugs aren’t hard to replace. Just look the engine over carefully to see if any have blown out. Sometimes there are some freeze plugs you can’t see because they are encased in the housing which bolts the transmission to the engine. Also Chickn probably took it for granted that you know, how to remove a partially blown out freeze plug. But, in case you don’t know, if you see a freeze plug which is partially blown out or leaking, you may be tempted to drive a screwdriver in between the freeze plug and the hole to pry it out. DON’T DO IT. The round hole the freeze plug fits into is a sealing surface and must remain pristinely smooth. You don’t want any gouges in it. If you have to remove a freeze plug, take a self-tapping screw (the kind with a hex head used to screw into metal - It will have a little drill bit head on the end of the screw) and use a screw gun to drill that into the middle of the freeze plug. Then you can get something like a hammer or nail puller on the end of that screw head and pull the freeze plug out.
It’s not useful to you now, but for future reference you may not be aware antifreeze not only prevents freeze-up but also helps with cooling the engine. Anti-freeze lowers the boiling point of the water so that the coolant doesn’t boil off as easily.
Antifreeze also serves as a rust/corrosion inhibitor. That may not sound like a big deal (after all, who ever heard of an engine block rusting out?) but internal engine corrosion is hard on water pumps and freeze plugs can rust through. Also proper anti-freeze can prevent cavitation in the water pump and cavitation around the cylinder walls of diesel engines. (Am I making you feel worse, yet?)
Seriously though, Derby, Chickn and Steve333 are right. All may not be lost. Freezing doesn’t always break things. Iron or copper pipes can sometimes freeze without breaking. I even freeze stuff in mason jars in my deep freeze sometimes.
Just make sure you get some anti-freeze in the tractor and let it warm up so it circulates everywhere as soon as is practical. Just because freezing doesn’t break things the first time or second time, doesn’t mean it won’t eventually break something from freezing/thawing repeatedly.
Lastly, just a reminder to anyone who hasn’t thought about it, that it’s a good idea to put some anti-freeze in any spray equipment you store outside. I’ve seen spray pumps broken because they froze. I’ve even lost a spray gun because of freezing, even though I thought I had removed all the water from the sprayer. Now I just put a little anti-freeze in the sprayer and run it through, in the fall. It only takes a little bit of anti-freeze.