Q: In a recent cold snap my garlic and winter squash harvest froze in my unheated garage. It remains frozen. Is this a bad thing? If so, what can I do to salvage/preserve what's left of it?
A: This situation is not ideal, Freeze Frame. Garlic stands a better chance of withstanding a freeze if it has already sprouted, such as the garlic you've (hopefully) planted for next year's harvest.
Your stored garlic and squash are now in danger of imminent decay. But the upside is that this decay will be suspended as long as they stay frozen.
I suggest keeping both garlic and squash frozen while you thaw a bit of each and assess the damage. In the case of the garlic, if the defrosted test-bulb appears unscathed—with firm, supple body and strong flavor—then slowly thaw the rest and don't let it freeze again. If the thawed test-garlic shows damage, your best option is to keep it frozen—that will at least stabilize the condition. Keep it frozen and thaw for immediate use as needed.
The same, basically, goes for the squash. Thaw one out and see if it appears like normal raw squash. If it's fine, give your brow a relieved wipe with the back of your hand, thaw the rest of your squash and don't let it freeze again.
But if the test squash thaws mushy (and you have the freezer space to keep it frozen if temperatures in your garage rise above freezing), then keep it frozen. If you don't have the freezer space, you should thaw the rest and cook it, either by steaming or baking. This will reduce the size, hopefully to a manageable amount. Freeze the cooked squash until you're ready to use it.
Because of the damage incurred by freezing, the squash won't be as versatile as it was. But even though it will have lost some of its body and texture—and you won't be able to stuff it or add chunks of it to your breakfast taco—the twice-frozen, once-cooked squash will still perform adequately in soup, pie or pureed mush on the side of your plate.